misc/cgo/test: fix test when using GCC 7

With GCC 7 (not yet released), cgo fails with errors like

./sigaltstack.go:65:8: call of non-function C.restoreSignalStack

I do not know precisely why. Explicitly declaring that there are no
arguments to the static function is a simple fix for the debug info.

Change-Id: Id96e1cb1e55ee37a9f1f5ad243d7ee33e71584ac
Reviewed-on: https://go-review.googlesource.com/35480
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/test/sigaltstack.go

@@ -17,7 +17,7 @@ package cgotest
 static stack_t oss;
 static char signalStack[SIGSTKSZ];
 
-static void changeSignalStack() {
+static void changeSignalStack(void) {
 	stack_t ss;
 	memset(&ss, 0, sizeof ss);
 	ss.ss_sp = signalStack;
@@ -29,7 +29,7 @@ static void changeSignalStack() {
 	}
 }
 
-static void restoreSignalStack() {
+static void restoreSignalStack(void) {
 #if (defined(__x86_64__) || defined(__i386__)) && defined(__APPLE__)
 	// The Darwin C library enforces a minimum that the kernel does not.
 	// This is OK since we allocated this much space in mpreinit,
@@ -42,7 +42,7 @@ static void restoreSignalStack() {
 	}
 }
 
-static int zero() {
+static int zero(void) {
 	return 0;
 }
 */