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

Commit 9183df25 ("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: NDavid Herrmann <dh.herrmann@gmail.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

include/uapi/asm-generic/unistd.h

@@ -703,9 +703,11 @@ __SYSCALL(__NR_renameat2, sys_renameat2)
 __SYSCALL(__NR_seccomp, sys_seccomp)
 #define __NR_getrandom 278
 __SYSCALL(__NR_getrandom, sys_getrandom)
+#define __NR_memfd_create 279
+__SYSCALL(__NR_memfd_create, sys_memfd_create)
 
 #undef __NR_syscalls
-#define __NR_syscalls 279
+#define __NR_syscalls 280
 
 /*
  * All syscalls below here should go away really,