lang/opensycl: fix build without libomp
CMake Error at /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:230 (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:600 (_FPHSA_FAILURE_MESSAGE) /usr/local/share/cmake/Modules/FindOpenMP.cmake:581 (find_package_handle_standard_args) src/runtime/CMakeLists.txt:181 (find_package)