syscalls: add __NR_kcmp syscall to generic unistd.h

Commit d97b46a6 ("syscalls, x86: add __NR_kcmp syscall" ) added a new
syscall to support checkpoint restore. It is currently x86-only, but
that restriction will be removed in a subsequent patch. Unfortunately,
the kernel checksyscalls script had a bug which suppressed any warning
to other architectures that the kcmp syscall was not implemented. A
patch to checksyscalls is being tested in linux-next and other
architectures are seeing warnings about kcmp being unimplemented.

This patch adds __NR_kcmp to <asm-generic/unistd.h> so that kcmp is
wired in for architectures using the generic syscall list.

Signed-off-by: Mark Salter <msalter@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>