EglSyncFence tests depend on android platform

If you build for MIR_PLATFORM=mesa-kms;mesa-x11 then you get build errors:

CMakeFiles/mir_unit_tests.dir/graphics/test_egl_sync_fence.cpp.o: In function `EglSyncFence_can_wait_for_fence_with_timeout_Test::TestBody()':
/«BUILDDIR»/mir-0.20.0+16.04.20160217.3/tests/unit-tests/graphics/test_egl_sync_fence.cpp:100: undefined reference to `mir::graphics::EGLSyncFence::EGLSyncFence(std::shared_ptr<mir::graphics::EGLSyncExtensions> const&)'
/«BUILDDIR»/mir-0.20.0+16.04.20160217.3/tests/unit-tests/graphics/test_egl_sync_fence.cpp:101: undefined reference to `mir::graphics::EGLSyncFence::raise()'
/«BUILDDIR»/mir-0.20.0+16.04.20160217.3/tests/unit-tests/graphics/test_egl_sync_fence.cpp:102: undefined reference to `mir::graphics::EGLSyncFence::wait_for(std::chrono::duration<long, std::ratio<1l, 1000000000l> >)'
/«BUILDDIR»/mir-0.20.0+16.04.20160217.3/tests/unit-tests/graphics/test_egl_sync_fence.cpp:100: undefined reference to `mir::graphics::EGLSyncFence::~EGLSyncFence()'
/«BUILDDIR»/mir-0.20.0+16.04.20160217.3/tests/unit-tests/graphics/test_egl_sync_fence.cpp:100: undefined reference to `mir::graphics::EGLSyncFence::~EGLSyncFence()'
CMakeFiles/mir_unit_tests.dir/graphics/test_egl_sync_fence.cpp.o: In function `EglSyncFence_can_clear_without_waiting_Test::TestBody()':
/«BUILDDIR»/mir-0.20.0+16.04.20160217.3/tests/unit-tests/graphics/test_egl_sync_fence.cpp:118: undefined reference to `mir::graphics::EGLSyncFence::EGLSyncFence(std::shared_ptr<mir::graphics::EGLSyncExtensions> const&)'
/«BUILDDIR»/mir-0.20.0+16.04.20160217.3/tests/unit-tests/graphics/test_egl_sync_fence.cpp:119: undefined reference to `mir::graphics::EGLSyncFence::raise()'
/«BUILDDIR»/mir-0.20.0+16.04.20160217.3/tests/unit-tests/graphics/test_egl_sync_fence.cpp:120: undefined reference to `mir::graphics::EGLSyncFence::reset()'
/«BUILDDIR»/mir-0.20.0+16.04.20160217.3/tests/unit-tests/graphics/test_egl_sync_fence.cpp:118: undefined reference to `mir::graphics::EGLSyncFence::~EGLSyncFence()'
/«BUILDDIR»/mir-0.20.0+16.04.20160217.3/tests/unit-tests/graphics/test_egl_sync_fence.cpp:118: undefined reference to `mir::graphics::EGLSyncFence::~EGLSyncFence()'
CMakeFiles/mir_unit_tests.dir/graphics/test_egl_sync_fence.cpp.o: In function `void __gnu_cxx::new_allocator<mir::graphics::EGLSyncExtensions>::construct<mir::graphics::EGLSyncExtensions>(mir::graphics::EGLSyncExtensions*)':
/usr/include/c++/5/bits/shared_ptr_base.h:518: undefined reference to `mir::graphics::EGLSyncExtensions::EGLSyncExtensions()'
/usr/include/c++/5/bits/shared_ptr_base.h:518: undefined reference to `mir::graphics::EGLSyncExtensions::EGLSyncExtensions()'
/usr/include/c++/5/bits/shared_ptr_base.h:518: undefined reference to `mir::graphics::EGLSyncExtensions::EGLSyncExtensions()'
/usr/include/c++/5/bits/shared_ptr_base.h:518: undefined reference to `mir::graphics::EGLSyncExtensions::EGLSyncExtensions()'
/usr/include/c++/5/bits/shared_ptr_base.h:518: undefined reference to `mir::graphics::EGLSyncExtensions::EGLSyncExtensions()'

https://launchpadlibrarian.net/240210508/buildlog_ubuntu-xenial-arm64.mir_0.20.0+16.04.20160217.3-0ubuntu1_BUILDING.txt.gz