Declare layer_count extern, so that plugins outside of core can use it too

Without declaring it extern, other plugins will not have access to the symbol.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

src/layers.h

@@ -27,6 +27,7 @@
   const Key keymaps[][ROWS][COLS] PROGMEM = { layers };		\
   uint8_t layer_count = sizeof(keymaps) / sizeof(*keymaps);
 
+extern uint8_t layer_count;
 
 class Layer_ {
  public: