Add -lkstat to the .pc for Solaris (google#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

cmake/benchmark.pc.in

@@ -8,5 +8,5 @@ Description: Google microbenchmark framework
 Version: @VERSION@
 
 Libs: -L${libdir} -lbenchmark
-Libs.private: -lpthread
+Libs.private: -lpthread @BENCHMARK_PRIVATE_LINK_LIBRARIES@
 Cflags: -I${includedir}

src/CMakeLists.txt

@@ -64,6 +64,7 @@ endif()
 # We need extra libraries on Solaris
 if(${CMAKE_SYSTEM_NAME} MATCHES "SunOS")
   target_link_libraries(benchmark PRIVATE kstat)
+  set(BENCHMARK_PRIVATE_LINK_LIBRARIES -lkstat)
 endif()
 
 if (NOT BUILD_SHARED_LIBS)