This fixes the following error received on OpenBSD when running `install.sh`.
```
$ sh install.sh
Copying fonts...
cp: cannot stat '%': No such file or directory
All Powerline fonts installed to /home/foo/.local/share/fonts
```
What's happening is that OpenBSD's `xargs` fails without the `-n1` option (i.e. copy the files one-at-a-time).
Adding the `-n1` seems to be compatible with Linux (I tried it on a CentOS system); probably holds for Mac OS X too (but I don't have one to test on).