version CMakeLists.txt: add ver_gen dependency
version CMakeLists.txt: add ver_gen dependency

Necessary so that build_git_version.h is generated before version.c is
compiled.

Error before:
../src/lib/version/version.c:152:31: error: 'PX4_GIT_TAG_STR' undeclared (first use in this function)
  return version_tag_to_number(PX4_GIT_TAG_STR);

krbeverx/Firmware,PX4/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,krbeverx/Firmware,dagar/Firmware,PX4/Firmware,PX4/Firmware,dagar/Firmware,jlecoeur/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,jlecoeur/Firmware,krbeverx/Firmware,dagar/Firmware,acfloria/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,PX4/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,krbeverx/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,acfloria/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,Aerotenna/Firmware,krbeverx/Firmware,Aerotenna/Firmware,dagar/Firmware,jlecoeur/Firmware,dagar/Firmware,mcgill-robotics/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,PX4/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,Aerotenna/Firmware