FAST: use glib hash table implementation to store FAST fields

hsearch_r() API is not available on Darwin (a.k.a. Mac OS X)
so this is causing a build regression. Since glib is supported
on all relevant platforms, let use that library for the moment.

Reported-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Marat Stanichenko <mstanichenko@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>