Fix up try_to_freeze() usage in arch/i386/kernel/signal.c

The parentheses were missing. Noted by Pavel Machek.
diff --git a/arch/i386/kernel/signal.c b/arch/i386/kernel/signal.c
index ac5b1e9..89ef7ad 100644
--- a/arch/i386/kernel/signal.c
+++ b/arch/i386/kernel/signal.c
@@ -608,7 +608,7 @@
 	if (!user_mode(regs))
 		return 1;
 
-	if (try_to_freeze)
+	if (try_to_freeze())
 		goto no_signal;
 
 	if (!oldset)