util/disk_cache: Use backward compatible st_mtime.

util/disk_cache: Use backward compatible st_mtime.

Fix Mac OS X build error.

  CC       libmesautil_la-disk_cache.lo
In file included from disk_cache.c:46:
./disk_cache.h:57:20: error: no member named 'st_mtim' in 'struct stat'
   *timestamp = st.st_mtim.tv_sec;
                ~~ ^

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99918
Fixes: 207e3a6e4b ("util/radv: move *_get_function_timestamp() to utils")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>