libgcrypt-devel needs to be added to XB install instructions

During Compilation:

-- Looking for ACL_USER - not found.
CMake Error at cmake/gcrypt.cmake:25 (MESSAGE):
  Cannot find gcrypt.h in /usr/include;/usr/local/include;/opt/local/include.
  You can use libgcrypt-config --cflags to get the necessary path and pass it
  to CMake with -DGCRYPT_INCLUDE_PATH=<path>
Call Stack (most recent call first):
  storage/innobase/xtrabackup/src/CMakeLists.txt:20 (FIND_GCRYPT)
[...]

Solution:
$ sudo yum install libgcrypt-devel

This to be added to:
- ./BUILD.XtraBackup.txt in XB 2.2 tree under RPM req's
   (In RPM-based distributions, you need to:...)
- Future 2.2 documentation