Tests: Fix RunCMake.ExcludeFromAll test program warning

Add a return value to `main()`.
diff --git a/Tests/RunCMake/ExcludeFromAll/main.c b/Tests/RunCMake/ExcludeFromAll/main.c
index 5047a34..f8b643a 100644
--- a/Tests/RunCMake/ExcludeFromAll/main.c
+++ b/Tests/RunCMake/ExcludeFromAll/main.c
@@ -1,3 +1,4 @@
 int main()
 {
+  return 0;
 }