config: git should ignore config files in config directory

Running a git clean on a xfstests tree causes it to remove the
config files for the given host. Make git ignore custom config
files in this directory so git clean won't completely trash the
local config needed to run xfstests.

Signed-off-by: Dave Chinner <david@fromorbit.com>

.gitignore

@@ -23,6 +23,9 @@
 /patches
 /.pc
 
+# custom config files
+/configs/*.config
+
 # ltp/ binaries
 /ltp/aio-stress
 /ltp/doio