Add lost MKS_BASE to pins.h (PR#2297)

Marlin/pins.h

@@ -112,6 +112,8 @@
   #include "pins_BAM_DICE_DUE.h"
 #elif MB(FELIX2)
   #include "pins_FELIX2.h"
+#elif MB(MKS_BASE)
+  #include "pins_MKS_BASE.h"
 #elif MB(99)
   #include "pins_99.h"
 #else