Code review comment for lp:~sense/notify-osd/fix-465801

These changes fix bug #465801 and have been tested and found to be working.
The variable dirname in src/log.c is now a const gchar* and is not freed. Instead of hardcoding the use of ~/.cache, now the function g_get_user_cache_dir() is used. Therefore <glib.h> is included in src/log.c