fix libimobiledevice formula for —HEAD build

Hello,

the build failed when installing libimobiledevice from HEAD

```
$ brew install libimobiledevice --HEAD
==> Cloning http://cgit.sukimashita.com/libimobiledevice.git
Updating /Library/Caches/Homebrew/libimobiledevice--git
==> ./configure --disable-silent-rules --prefix=/usr/local/Cellar/libimobiledevice/HEAD --without-cython
Failed to execute: ./configure

READ THIS: https://github.com/Homebrew/homebrew/wiki/troubleshooting
```

This is a fix for the libimobiledevice Formula