libcdecl: Silence GCC -Wunused-function warnings in the scanner.

libcdecl: Silence GCC -Wunused-function warnings in the scanner.

Use the GCC unused attribute to avoid warnings in the scanner, and use
the flex yyIN_HEADER macro so the static declarations don't leak out to
other files (where GCC will give warnings about missing definitions).

While we do this, fix a typo and remove an unneeded declaration which
were both caught by this warning.