[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
```

Author: RoboTux

mlir/lib/Dialect/OpenMP/CMakeLists.txt

@@ -5,6 +5,7 @@ add_mlir_dialect_library(MLIROpenMPDialect
   ${MLIR_MAIN_INCLUDE_DIR}/mlir/Dialect/OpenMP
 
   DEPENDS
+  omp_gen
   MLIROpenMPOpsIncGen
   MLIROpenMPOpsInterfacesIncGen
   MLIROpenMPTypeInterfacesIncGen