[flang][nfc] Add missing dependency in CMake

This missing dependency has been causing the Flang buildbots
(with BUILD_SHARED_LIBS set to ON) to fail:
  * http://lab.llvm.org:8011/#/builders/66/builds/542
  * http://lab.llvm.org:8011/#/builders/33/builds/764

This missing dependency was exposed by this change:
  * https://reviews.llvm.org/D91461
This change is fine - the root cause of the failing builds is the
missing dependency.