Fix up prototype for sys_ipc breakage

Commit 45575f5a

 ("ppc64 sys_ipc breakage in 2.6.34-rc2") fixed the
definition of the sys_ipc() helper, but didn't fix the prototype in
<linux/syscalls.h>

Reported-and-tested-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>