libtorrent.pc.in: add Libs.Private

[ffontaine]

Add Libs.Private: -lz so applications that want to link statically with libtorrent (such as rtorrent) will know that they must link with -lz

Fixes:

• http://autobuild.buildroot.org/results/075598e1699c2ac20a4dfbcb5695bbb7343f9a86

Signed-off-by: Fabrice Fontaine fontaine.fabrice@gmail.com