graphics/luminance-qt5: fix build without libomp

CMake Error at /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:233 (message):
  Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES)
Call Stack (most recent call first):
  /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:603 (_FPHSA_FAILURE_MESSAGE)
  /usr/local/share/cmake/Modules/FindOpenMP.cmake:616 (find_package_handle_standard_args)
  build_files/Modules/CompilerSettings.cmake:40 (FIND_PACKAGE)
  CMakeLists.txt:38 (include)