xtensa: fix build failure in xtensa/kernel/signal.c

Caused by commit 3785006a

    "xtensa: don't mask signals if we fail to setup signal stack"

It assigns a return value to "ret", but there is no such variable
anywhere in scope.  Create one.

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Chris Zankel <chris@zankel.net>
Acked-by: Matt Fleming <matt.fleming@intel.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

arch/xtensa/kernel/signal.c

@@ -496,6 +496,7 @@ int do_signal(struct pt_regs *regs, sigset_t *oldset)
 	signr = get_signal_to_deliver(&info, &ka, regs, NULL);
 
 	if (signr > 0) {
+		int ret;
 
 		/* Are we from a system call? */