cmake: ensure generated source files exist before extracting events

Only needed for Makefile-based builds:
gmake[3]: *** No rule to make target 'src/modules/flight_mode_manager/FlightTasks_generated.hpp', needed by 'events/px4.json'.  Stop.

src/systemcmds/topic_listener/CMakeLists.txt

@@ -33,15 +33,15 @@
 
 add_custom_command(OUTPUT listener_generated.cpp
 	COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/generate_listener.py ${PX4_SOURCE_DIR} ${msg_files} > listener_generated.cpp
-	DEPENDS generate_listener.py uorb_msgs
+	DEPENDS generate_listener.py ${msg_files}
 )
 
 add_custom_target(generate_topic_listener
 	DEPENDS
 		listener_generated.cpp
 		generate_listener.py
-		uorb_msgs
 )
+add_dependencies(prebuild_targets generate_topic_listener)
 
 px4_add_module(
 	MODULE systemcmds__topic_listener
@@ -56,5 +56,6 @@ px4_add_module(
 		${CMAKE_CURRENT_BINARY_DIR}/listener_generated.cpp
 	DEPENDS
 		generate_topic_listener
+		uorb_msgs
 	)