Fix static_lexer.hpp generation.
Fix static_lexer.hpp generation.

Apparently I broke generation of static_lexer.hpp two months ago and
haven't noticed since I haven't changed the lexer and static_lexer.hpp
existed from a previous generation.

This fixes generation by adding a "generate files" target which
libpuppet depends on, which in turn depends out the output of the custom
command.

peterhuene/puppetcpp,iankronquist/puppetcpp