[flang] Fix issue of flang/runtime/config.h not being found in out of tree builds

Summary: Fixes https://bugs.llvm.org/show_bug.cgi?id=46078

Reviewers: DavidTruby, jdoerfert, PeteSteinfeld, sscalpone, tskeith

Reviewed By: PeteSteinfeld, sscalpone, tskeith

Subscribers: mgorny, llvm-commits, tskeith

Tags: #llvm, #flang

Differential Revision: https://reviews.llvm.org/D81266