misc/cgo/testsigfwd: add missing return statement

Fixes C compiler warning:

./main.go:54:1: warning: control reaches end of non-void function [-Wreturn-type]

Should help fix the linux builders
that broke due to CL 23005.

Change-Id: Ib0630798125e35a12f99d666b7ffe7b3196f0ecc
Reviewed-on: https://go-review.googlesource.com/28176
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/testsigfwd/main.go

@@ -50,6 +50,7 @@ static void iohandler(int signum) {
 
 static void* sigioThread(void* arg __attribute__ ((unused))) {
 	raise(SIGIO);
+	return NULL;
 }
 
 static void sigioOnThread() {