[llvm] [flang][CMake] Add missing dependency to generate Fortran module files (PR #91517)

[Michael Klemm via llvm-commits]

mjklemm wrote:

The issue was present in runtime builds of OpenMP, but was more like a race condition with some succesful builds and some failing.  Esp. for stage2 builds it failed more consistently.  I guess regular project build were not affected, as all targets had been build in order.

https://github.com/llvm/llvm-project/pull/91517