Add -lpthread to pkg-config Libs.private. (#755)

Since pthread is required at least for GCC (according to the
documentation), this should be reflected by the pkg-config file.

The same is, for instance, also done by the gflags library:
1005485222/cmake/package.pc.in (L13)

cmake/benchmark.pc.in

@@ -8,4 +8,5 @@ Description: Google microbenchmark framework
 Version: @VERSION@
 
 Libs: -L${libdir} -lbenchmark
+Libs.private: -lpthread
 Cflags: -I${includedir}