asm-generic: add memfd_create system call to unistd.h

asm-generic: add memfd_create system call to unistd.h

Commit 9183df25fe7b ("shm: add memfd_create() syscall") added a new
system call (memfd_create) but didn't update the asm-generic unistd
header.

This patch adds the new system call to the asm-generic version of
unistd.h so that it can be used by architectures such as arm64.

Cc: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Change-Id: I7fff684716a86ad9f10e19755480c32ce9eeb861
Origin: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=503e6636b6f96056210062be703356f4253b6db9
Backported-by: Maciej Wereski <m.wereski@partner.samsung.com>
Signed-off-by: Maciej Wereski <m.wereski@partner.samsung.com>