[flang][driver] Add missing parentheses in an assert

The assert in https://reviews.llvm.org/D124665 was missing parentheses,
which triggered a warning in GCC (verified with GCC 11). As `-Werror` is
on by default in FLang, that triggered build errors, see e.g. [1].

The fix is rather straightforward, so I am sending this without a
review.

[1] https://lab.llvm.org/buildbot/#/builders/160/builds/7016

Differential Revision: https://reviews.llvm.org/D125027