third_party: Add gpfs.h header file

third_party: Add gpfs.h header file

This is the only requirement for building the vfs_gpfs module; the
corresponding library is opened through dlopen at runtime. The intent
here is to always build the vfs_gpfs module to easily detect breakage
e.g. due to vfs interface changes.

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>