Only trigger encoder callbacks on primary side (#18467)

Co-authored-by: zvecr <git@zvecr.com>

quantum/encoder/tests/mock_split.c

@@ -36,3 +36,7 @@ bool setPin(pin_t pin, bool val) {
 }
 
 void last_encoder_activity_trigger(void) {}
+
+__attribute__((weak)) bool is_keyboard_master(void) {
+    return true;
+}