This fixes the following failure when doing a clean build (in particular no .ninja* lying around) of lib/libMLIROpenMPDialect.a only: ``` In file included from mlir/lib/Dialect/OpenMP/IR/OpenMPDialect.cpp:29: llvm/include/llvm/Frontend/OpenMP/OMPConstants.h:20:10: fatal error: llvm/Frontend/OpenMP/OMP.h.inc: No such file or directory ```