Fix compilation of examples

src/examples/attitude_estimator_ekf/CMakeLists.txt

src/examples/ekf_att_pos_estimator/CMakeLists.txt

src/examples/mc_att_control_multiplatform/CMakeLists.txt

src/examples/mc_pos_control_multiplatform/CMakeLists.txt

src/examples/mc_pos_control_multiplatform/mc_pos_control.cpp