[flang][nfc] Add missing REQUIRES directive in a test

Failing bot:
  * https://lab.llvm.org/buildbot/#/builders/21/builds/88731

Failing test was introduced in:
  * https://github.com/llvm/llvm-project/pull/74598

Sending without a review as the fix is straightforward and I want to
prioritize fixing the broken bot and unblocking everyone who's been
affected.