[MLIR] Add missing omp_gen dep to MLIROpenMPDialect (#84552)

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
```