build: Stop putting FFI_CFLAGS in AM_CFLAGS

AM_CFLAGS are the defaults passed to anything that doesn't specify its own
_CFLAGS, so instead of putting FFI_CFLAGS there, let's just add that to
anything that actually needs it.

The only thing that needs it but didn't have it specifically was
libwayland_util (for connection.c)

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>