gapi(test): avoid anonymous namespace types as template parameters

modules/gapi/test/cpu/gapi_ocv_stateful_kernel_test_utils.hpp

@@ -22,8 +22,10 @@ struct Name                                      \
 
 namespace opencv_test
 {
-namespace
-{
+
+// types from anonymous namespace doesn't work well with templates
+inline namespace gapi_ocv_stateful_kernel_test_utils {
+
 struct UserStruct
 {
     UserStruct() = default;
@@ -41,7 +43,8 @@ private:
     short _myShortVal;
     float _myFloatVal;
 };
-} // anonymous namespace
+
+} // namespace
 } // opencv_test
 
 #endif // OPENCV_GAPI_OCV_STATEFUL_KERNEL_TESTS_UTILS_HPP