[openmp] Add a dependency on the separate import library (#80449)

Currently, when doing e.g. "ninja check-openmp", the check-openmp target
only depends on the target "omp", which builds the library. Thus by
doing that, the separate import library "libomp.lib", which is generated
directly from a def file, never gets created, unless one does a separate
invocation first, that builds all targets.

To fix this, make the "omp" target depend on the target for the separate
import library, whenever that is created/used.