misc/cgo/test: fix build for CC=clang

Fix build error when CL=clang introduced by CL 10173.

Change-Id: I8edf210787a9803280c0779ff710c7e634a820d6
Reviewed-on: https://go-review.googlesource.com/10341Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/test/sigprocmask_linux.c

@@ -22,6 +22,7 @@ static void* sigthreadfunc(void* unused) {
 	sigaddset(&mask, SIGIO);
 	sigprocmask(SIG_BLOCK, &mask, NULL);
 	IntoGoAndBack();
+	return NULL;
 }
 
 int RunSigThread() {