Fix typo so that ninja build works

[Ninja](http://martine.github.io/ninja) is an alternative to make.
```
Ninja is a small build system with a focus on speed.
It differs from other build systems in two major respects:
it is designed to have its input files generated by a higher-level build system,
and it is designed to run builds as fast as possible.
```
Then libzypp can be built as as:

```
sudo zypper install ninja
cd build
cmake -G Ninja ..
ninja
```