Fix build on big-endian

Commit 50d1285b updated the wl_shm enum values but left
one out. Update it to fix the build on big-endian.

https://bugzilla.gnome.org/show_bug.cgi?id=710135

Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 9246286846d0428b03e646b2f5ec14f0eff4edc6)