benchmark

Add -lkstat to the .pc for Solaris (#1801)

* Add -lkstat to the .pc for Solaris

This fixes linking for projects that rely on pkg-config to generate the
link line on Solaris.

Test plan: Built the project locally on Solaris and verified -kstat
appears in the .pc file

```
$ cat lib/pkgconfig/benchmark.pc  | grep Libs.private
Libs.private: -lpthread -lkstat
```

* Use BENCHMARK_PRIVATE_LINK_LIBRARIES