Make `LayerCount` available in layers.cpp

src/layers.h

@@ -4,6 +4,9 @@
 #include "key_defs.h"
 #include KALEIDOSCOPE_HARDWARE_H
 
+// The total number of defined layers in the firmware sketch keymaps[] array
+extern const uint8_t LayerCount;
+
 class Layer_ {
  public:
   Layer_(void);