Add *.sch-bak to KiCad.gitignore (#3153)

Newer versions of KiCad call the schematics backup file `.sch-bak`.

KiCad.gitignore

@@ -6,6 +6,7 @@
 *.bak
 *.bck
 *.kicad_pcb-bak
+*.sch-bak
 *~
 _autosave-*
 *.tmp