KiCad: add footprint cache (#3043)

As described in commit c5a2ea1dd8b8f85f7524bbdaa17844f2324d02c5 in the
kicad repository itself, the fp-info-cache file should not be checked
in to version control.

(See https://git.launchpad.net/kicad/commit/?id=c5a2ea1dd8b8f85f7524bbdaa17844f2324d02c5)