shm: add memfd.h to UAPI export list

The new header file memfd.h from commit 9183df25

 ("shm: add
memfd_create() syscall") should be exported.

Signed-off-by: David Drysdale <drysdale@google.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>