KiCad: add footprint cache (#3043)

As described in commit c5a2ea1dd8 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)

KiCad.gitignore

 *-save.pro
 *-save.kicad_pcb
 
+# Footprint cache
+fp-info-cache
+
 # Netlist files (exported from Eeschema)
 *.net