Otherwise the definitions determined by CMake are not available to
opm/simulators/linalg/bda/opencl.hpp and choosing the correct header
might fail.
On my system this at least fixes a warning.
Fallout from #3749
Kernel files are located in opm/simulators/linalg/bda/opencl/kernels.
CMake will combine them for usage in
${PROJECT_BINARY_DIR}/clSources.cpp that becomes part of the library.