IOS/USB_KBD: Make IsKeyPressed() a non-virtual const member function

No other class inherits the USB_KBD class, and this function doesn't
actually modify instance state, so it can be made a const member
function.

Source/Core/Core/IOS/USB/USB_KBD.cpp

@@ -219,13 +219,10 @@ IPCCommandResult USB_KBD::IOCtl(const IOCtlRequest& request)
   return GetDefaultReply(IPC_SUCCESS);
 }
 
-bool USB_KBD::IsKeyPressed(int _Key)
+bool USB_KBD::IsKeyPressed(int key) const
 {
 #ifdef _WIN32
-  if (GetAsyncKeyState(_Key) & 0x8000)
-    return true;
-  else
-    return false;
+  return (GetAsyncKeyState(key) & 0x8000) != 0;
 #else
   // TODO: do it for non-Windows platforms
   return false;

Source/Core/Core/IOS/USB/USB_KBD.h

@@ -54,7 +54,7 @@ private:
   std::array<bool, 256> m_OldKeyBuffer{};
   u8 m_OldModifiers = 0;
 
-  virtual bool IsKeyPressed(int _Key);
+  bool IsKeyPressed(int key) const;
 
   // This stuff should probably die
   enum