Make LED modes automatically use LEDControl

This way the end-user does not have to explicitly call
`Keyboardio.use(&LEDControl)`, it is enough to use a LED effect.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

src/Keyboardio-LEDControl.cpp

@@ -10,6 +10,7 @@ LEDMode::activate (void) {
 
 void
 LEDMode::begin(void) {
+  Keyboardio.use(&LEDControl, NULL);
   LEDControl.mode_add(this);
 }