cmake only include mavlink where used

cmake/common/px4_base.cmake

cmake/posix/px4_impl_posix.cmake

cmake/qurt/px4_impl_qurt.cmake

src/drivers/pwm_out_rc_in/CMakeLists.txt

src/drivers/snapdragon_rc_pwm/CMakeLists.txt

src/modules/mavlink/CMakeLists.txt

src/modules/mavlink/mavlink_tests/CMakeLists.txt

src/modules/simulator/CMakeLists.txt

src/modules/uavcan/CMakeLists.txt

src/platforms/posix/drivers/accelsim/CMakeLists.txt

src/platforms/posix/drivers/airspeedsim/CMakeLists.txt

src/platforms/posix/drivers/barosim/CMakeLists.txt

src/platforms/posix/drivers/gpssim/CMakeLists.txt

src/platforms/posix/drivers/gyrosim/CMakeLists.txt