s3:wscript: vfs_gpfs needs kernel oplock support

s3:wscript: vfs_gpfs needs kernel oplock support

It uses symbols, which are only available if we have
HAVE_KERNEL_OPLOCKS_LINUX defined.

This is not the case when building withing the
Windows Subsystem for Liux (WSL). So we better don't try to
build the vfs_gpfs module there.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Tue Jul  7 09:37:37 UTC 2020 on sn-devel-184