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:
https://github.com/gflags/gflags/blob/1005485222e8b0feff822c5723ddcaa5abadc01a/cmake/package.pc.in#L13