Remove Requires.private: zlib on FreeBSD <10.

On FreeBSD 9, the 'Requires.private: zlib' line in tcl.pc causes breakage in
ports that use pkg-config tcl to build their cflags, libraries, etc. This
commit fixes at least the devel/libdistance build on 9; it may fix others, too.

Approved by: gahr
Differential Revision: https://reviews.freebsd.org/D4359