include/linux/compat.h: declare compat_sys_sendmmsg()

This is required for tilegx to be able to use the compat unistd.h header
where compat_sys_sendmmsg() is now mentioned.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>