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.

msg/CMakeLists.txt

@@ -34,6 +34,8 @@
 # Support IN_LIST if() operator
 cmake_policy(SET CMP0057 NEW)
 
+include(px4_list_make_absolute)
+
 set(msg_files
 	actuator_armed.msg
 	actuator_controls.msg
@@ -230,6 +232,8 @@ if(invalid_msgs)
 	endif()
 endif()
 
+px4_list_make_absolute(msg_files ${CMAKE_CURRENT_SOURCE_DIR} ${msg_files})
+
 if(NOT EXTERNAL_MODULES_LOCATION STREQUAL "")
 	# Check that the msg directory and the CMakeLists.txt file exists
 	if(EXISTS ${EXTERNAL_MODULES_LOCATION}/msg/CMakeLists.txt)