Add `.clang_complete` and `Xdummy.so` to gitignore

Long description:

Adds two files to gitignore.

`.clang_complete` is created by the clang_complete vim plugin
[https://github.com/Rip-Rip/clang_complete]

`Xdummy.so` is created by compiling the Xdummy module.

To apply this patch, use:
curl http://cr.i3wm.org/patch/282/raw.patch | git am

@@ -31,3 +31,4 @@ docs/*.html

 !/docs/refcard.html

 i3-command-parser.stamp

 i3-config-parser.stamp

+.clang_complete

@@ -8,3 +8,4 @@ inc

 META.yml

 i3-cfg-for-*

 -

+Xdummy.so