Add returns to constexpr tests

These are needed since main() is actually replaced with SDL_main()

tests/test_color_constexpr.cc

@@ -24,4 +24,6 @@ int main(int, char*[]) {
 	constexpr int a = c1.GetAlpha();
 
 	static_assert(r == 0 && g == 0 && b == 0 && a == 0, "");
+
+	return 0;
 }

tests/test_pointrect_constexpr.cc

@@ -77,4 +77,6 @@ int main(int, char*[]) {
 
 		static_assert(b3 && b4 && b5 && b6, "");
 	}
+
+	return 0;
 }