[flang][NFC] Restore I/O runtime API header name (#132423)
flang/include/flang/Runtime/io-api.h was changed into io-api-consts.h,
then wrapped into a new io-api.h that includes io-api-consts.h, does
some redundant includes and declarations, and then declares the
prototype of one function, InquiryKeywordHashDecode.
Make that function static in io-stmt.cpp prior to its sole call site,
then undo the renaming, to reduce confusion and redundancy.
