Add override keyword through the code.

This was automated using `clang-modernize`.

src/core/hle/service/fs.h

@@ -23,7 +23,7 @@ public:
      * Gets the string port name used by CTROS for the service
      * @return Port name of service
      */
-    std::string GetPortName() const {
+    std::string GetPortName() const override {
         return "fs:USER";
     }
 };