pulse_counter_ulp: Add required structure to dummy header

esphome/components/pulse_counter_ulp/ulp/ulp_main.h

@@ -1,3 +1,7 @@
+#pragma once
+
 // Dummy file to appease clang-tidy
 // The real file is generated automatically by the IDF esp32ulp_mapgen.py
 // utility
+
+namespace pulse_counter_ulp {}