cmake: add missing fix_omp.h and fix_omp.cpp to compilation

    set(USER-OMP_SOURCES_DIR ${LAMMPS_SOURCE_DIR}/USER-OMP)

    set(USER-OMP_SOURCES ${USER-OMP_SOURCES_DIR}/thr_data.cpp

                         ${USER-OMP_SOURCES_DIR}/thr_omp.cpp

                         ${USER-OMP_SOURCES_DIR}/fix_omp.cpp

                         ${USER-OMP_SOURCES_DIR}/fix_nh_omp.cpp

                         ${USER-OMP_SOURCES_DIR}/fix_nh_sphere_omp.cpp

                         ${USER-OMP_SOURCES_DIR}/domain_omp.cpp)

    # detects styles which have USER-OMP version

    RegisterStylesExt(${USER-OMP_SOURCES_DIR} omp OMP_SOURCES)

    RegisterFixStyle("${USER-OMP_SOURCES_DIR}/fix_omp.h")

    get_property(USER-OMP_SOURCES GLOBAL PROPERTY OMP_SOURCES)

    AddStyleHeader(${path} NPAIR)

endfunction(RegisterNPairStyle)

function(RegisterFixStyle path)

    AddStyleHeader(${path} FIX)

endfunction(RegisterFixStyle)

function(RegisterStyles search_path)

    FindStyleHeaders(${search_path} ANGLE_CLASS     angle_     ANGLE     ) # angle     ) # force

    FindStyleHeaders(${search_path} ATOM_CLASS      atom_vec_  ATOM_VEC  ) # atom      ) # atom      atom_vec_hybrid