[flang][driver] Add missing dependency in unit tests (nfc)

The following patch revealed a missing dependency in the CMake script
for Flang driver unit tests:
  * https://reviews.llvm.org/D96032
The following buildbots are failing ("BUILD_SHARED_LIBS" is set to ON):
  * http://lab.llvm.org:8011/#/builders/134/builds/1840
  * http://lab.llvm.org:8011/#/builders/66/builds/1785
  * http://lab.llvm.org:8011/#/builders/33/builds/2436

This patch adds the missing dependency.

From what I can see, FortranSemantics and FortranParser are only
indirect dependencies of FlangFrontendTests and IIUC shouldn't be
required here. This is something that we should revisit at some point.
In this patch I focus on fixing the build.