Fix unused-function warnings in lex output

Fix the unused-function warnings in the output of lex. In particular, we
silence a warning about yyunput being static but unused by exposing it as
a global function. This costs us a few hundred bytes in the binary but
the other available solutions are flex-specific or more complicated.
YY_NO_UNPUT used to silence this but no longer works in recent flex.
On the other hand YY_NO_INPUT does work to silence warnings about input().

Signed-off-by: Peter Maydell <pmaydell@chiark.greenend.org.uk>