x86: syscalls.h: remove asmlinkage from declaration of sys_rt_sigreturn()

Impact: cleanup

asmlinkage for sys_rt_sigreturn() no longer exists in arch/x86/kernel/signal.c.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>