autoconf: Add config.cache to .gitignore

Add generated file config.cache (default cache file, when running
./configure with -C, --config-cache option) to the list of ignored
files.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

.gitignore

@@ -141,6 +141,7 @@ git-core.spec
 *.py[co]
 config.mak
 autom4te.cache
+config.cache
 config.log
 config.status
 config.mak.autogen