syscalls.h add the missing sys_pipe2 declaration

In order to build the generic syscall table, we need a declaration for
every system call.  sys_pipe2 was added without a proper declaration, so
add this to syscalls.h now.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>