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/10341
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/misc/cgo/test/sigprocmask_linux.c b/misc/cgo/test/sigprocmask_linux.c
index 6597e98..518c533 100644
--- a/misc/cgo/test/sigprocmask_linux.c
+++ b/misc/cgo/test/sigprocmask_linux.c
@@ -22,6 +22,7 @@
 	sigaddset(&mask, SIGIO);
 	sigprocmask(SIG_BLOCK, &mask, NULL);
 	IntoGoAndBack();
+	return NULL;
 }
 
 int RunSigThread() {