glib: xz is a build time dependency.

Installing glib from a bottle failed for me due to a sha mismatch. This reverts to building from
source which is fine. However that fails because xz is not installed which is a build time dependency. 
This should fix that.   

```
Error: SHA1 mismatch
Expected: bd656d91c1641f0f12a0c509c60a7cb7b10416de
Actual: 290402d900e69daeb9a53d6ee5be0941702c5c20
Archive: /Library/Caches/Homebrew/glib-2.38.2.mountain_lion.bottle.tar.gz
To retry an incomplete download, remove the file above.
Warning: Bottle installation failed: building from source.
Error: /usr/local/opt/xz not present or broken
Please reinstall xz. Sorry :(
```