gh-112305: Fix check-clean-src to detect frozen_modules .h files. (#113344)

A typo left this check broken so many of us who do out-of-tree builds
were seeing strange failures due to bad `Python/frozen_modules/*.h`
files being picked up from the source tree and used at build time from
different Python versions leading to errors like:

`Fatal Python error: _PyImport_InitCore: failed to initialize importlib`

Or similar once our build got to an "invoke the interpreter"
bootstrapping step due to incorrect bytecode being embedded.