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/modules/flight_mode_manager/CMakeLists.txt

@@ -99,6 +99,9 @@ add_custom_command(
 	WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
 	VERBATIM
 )
+add_custom_target(flighttasks_generated
+	DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/FlightTasks_generated.cpp)
+add_dependencies(prebuild_targets flighttasks_generated)
 
 add_compile_options(
 	-Wno-cast-align
@@ -123,6 +126,7 @@ px4_add_module(
 	DEPENDS
 		px4_work_queue
 		WeatherVane
+		flighttasks_generated
 )
 
 # add all flight task dependencies