shm: add memfd.h to UAPI export list

shm: add memfd.h to UAPI export list

The new header file memfd.h from commit 9183df25fe7b ("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>
Change-Id: Ibcd915aad320ddedcfcca0b7a098e03cc883fd88
Origin: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=b01d072065b6f36550f486fe77f05b092225ba1b
Backported-by: Maciej Wereski <m.wereski@partner.samsung.com>
Signed-off-by: Maciej Wereski <m.wereski@partner.samsung.com>