CMakeLists.txt: fix cmake warning
CMakeLists.txt: fix cmake warning

It implicitly converted STRINGS to STRING.

mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,krbeverx/Firmware,dagar/Firmware,dagar/Firmware,PX4/Firmware,PX4/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,dagar/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,dagar/Firmware,krbeverx/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,dagar/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,acfloria/Firmware