Improve dependencies of distortos-doxygen CMake target
Improve dependencies of distortos-doxygen CMake target

See chapter "5. File-level dependencies of custom targets are not
propagated" at
https://samthursfield.wordpress.com/2015/11/21/cmake-dependencies-between-targets-and-files-and-custom-commands/

CezaryGapinski/distortos,CezaryGapinski/distortos,CezaryGapinski/distortos,jasmin-j/distortos,DISTORTEC/distortos,CezaryGapinski/distortos,DISTORTEC/distortos,jasmin-j/distortos,jasmin-j/distortos,CezaryGapinski/distortos,DISTORTEC/distortos,jasmin-j/distortos,DISTORTEC/distortos,jasmin-j/distortos