Update pins.h to include the Formbot board

Marlin/src/pins/pins.h

   #include "pins_AZTEEG_X3_PRO.h"     // ATmega2560                                 env:megaatmega2560
 #elif MB(ULTIMAIN_2)
   #include "pins_ULTIMAIN_2.h"        // ATmega2560                                 env:megaatmega2560
+#elif MB(FORMBOT)
+  #include "pins_FORMBOT.h"           // ATmega2560                                 env:megaatmega2560
 #elif MB(RUMBA)
   #include "pins_RUMBA.h"             // ATmega2560                                 env:megaatmega2560
 #elif MB(BQ_ZUM_MEGA_3D)