[PGO] Fix `instrprof-api.c` on Windows (#77508)

https://github.com/llvm/llvm-project/pull/76471 introduced a new test
but the check lines have over-restrictive patterns for a string variable
name that cause test failures on Windows (e.g.
https://lab.llvm.org/buildbot/#/builders/127/builds/60637/steps/4/logs/stdio).
This PR fixes the test.