Fixes unused variable in scripthelper.cpp.

extern/angelscript_addons/scripthelper/scripthelper.cpp

@@ -975,7 +975,7 @@ string ScriptGetExceptionInfo()
 
 void RegisterExceptionRoutines(asIScriptEngine *engine)
 {
-	int r;
+        [[maybe_unused]] int r;
 
 	// The string type must be available
 	assert(engine->GetTypeInfoByDecl("string"));