fix autohint

ttfautohint uses a noop hinter for glyphs in the private use area
http://www.freetype.org/ttfautohint/doc/ttfautohint.html#scripts
unless you specify the fallback option:
http://www.freetype.org/ttfautohint/doc/ttfautohint.html#fallback-script

This is suggested in the documentation for --symbol "Use this option
(usually in combination with option --latin-fallback) to hint symbol"

Before this change if you opened the generated font in FontForge
and looked at a glyph then Hint > Edit Instructions there would be
no instructions, after adding this flag, the glyph will have instructions
added.