build: fix/eliminate MSVC warnings

modules/gapi/include/opencv2/gapi/ocl/goclkernel.hpp

@@ -153,6 +153,10 @@ struct tracked_cv_umat{
     }
 };
 
+#ifdef _MSC_VER
+#pragma warning(push)
+#pragma warning(disable: 4702)  // unreachable code
+#endif
 template<typename... Outputs>
 void postprocess_ocl(Outputs&... outs)
 {
@@ -166,6 +170,9 @@ void postprocess_ocl(Outputs&... outs)
     int dummy[] = { 0, (validate(&outs), 0)... };
     cv::util::suppress_unused_warning(dummy);
 }
+#ifdef _MSC_VER
+#pragma warning(pop)
+#endif
 
 template<class T> struct ocl_get_out;
 template<> struct ocl_get_out<cv::GMat>