vfs_glusterfs: include glusterfs/api/glfs.h without relying on -I options

vfs_glusterfs: include glusterfs/api/glfs.h without relying on -I options

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13125

The glfs.h header file has always resided under glusterfs/api/ in the
standard include directory. The glusterfs-api.pc file adds the unneeded
-I${includedir}/glusterfs compiler option. This option will be removed
from future versions of the pkg-config file.

This change can safely be backported to older versions if there is a
need to have them build against glusterfs-3.13 or newer.

URL: https://review.gluster.org/18576
CC: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>
Autobuild-User(master): Günther Deschner <gd@samba.org>
Autobuild-Date(master): Thu Nov  9 22:37:30 CET 2017 on sn-devel-144