Make Marlin actually compile when #define REPRAPWORLD_KEYPAD is uncommented

Marlin/ultralcd.cpp

       } } while(0)
 
   /** Used variables to keep track of the menu */
-  #ifndef REPRAPWORLD_KEYPAD
-    volatile uint8_t buttons; // Bits of the pressed buttons.
-  #else
-    volatile uint8_t buttons_reprapworld_keypad; // The reprapworld_keypad shift register values
+  volatile uint8_t buttons;  //the last checked buttons in a bit array.
+  #ifdef REPRAPWORLD_KEYPAD
+    volatile uint8_t buttons_reprapworld_keypad; // to store the keypad shift register values
   #endif
+    
   #ifdef LCD_HAS_SLOW_BUTTONS
     volatile uint8_t slow_buttons; // Bits of the pressed buttons.
   #endif

Marlin/ultralcd_implementation_hitachi_HD44780.h

 * When selecting the Russian language, a slightly different LCD implementation is used to handle UTF8 characters.
 **/
 
-#ifndef REPRAPWORLD_KEYPAD
-  extern volatile uint8_t buttons;  //the last checked buttons in a bit array.
-#else
-  extern volatile uint16_t buttons;  //an extended version of the last checked buttons in a bit array.
-#endif
+//#ifndef REPRAPWORLD_KEYPAD
+//  extern volatile uint8_t buttons;  //the last checked buttons in a bit array.
+//#else
+  extern volatile uint8_t buttons;  //an extended version of the last checked buttons in a bit array.
+//#endif
 
 ////////////////////////////////////
 // Setup button and encode mappings for each panel (into 'buttons' variable