HID_: Make methods overridden from base-class explicit

Fixes abratchik#20. Done to make it clear that these virtual methods are defined in the PluggableUSBModule base-class and might be called from external code not seen in this repo.

src/HID/HID.h

@@ -133,10 +133,10 @@ class HID_ : public PluggableUSBModule
 
 protected:
     // Implementation of the PluggableUSBModule
-    int getInterface(uint8_t* interfaceCount);
-    int getDescriptor(USBSetup& setup);
-    bool setup(USBSetup& setup);
-    uint8_t getShortName(char* name);
+    int getInterface(uint8_t* interfaceCount) override;
+    int getDescriptor(USBSetup& setup) override;
+    bool setup(USBSetup& setup) override;
+    uint8_t getShortName(char* name) override;
 
 private:
     uint8_t epType[2];