If librepo is built with zchunk support, zchunk's zck.h header file is
included from librepo public header files, thus applications building
against librepo needs zchunk header files available at build time.
Those who only use pkg-config for tracking dependencies, reported
build failures:

    /usr/include/librepo/downloadtarget.h:27:10: fatal error: zck.h: No such file or directory

This patch adds the dependency on zck to librepo.pc.

It enhances Requires.private instead of Requires to prevent from
contaminating "pkgconf --libs librepo" with -lzck. Contrary, "pkgconf
--cflags librepo" correctly appends include path to zck header files.
The dependency on zck.pc is only checked and enforced with "pkgconf
--cflags librepo" invocation. This behaviour is a feature. See
<https://github.com/pkgconf/pkgconf/issues/352> and
<https://github.com/pkgconf/pkgconf/issues/300>.

Fixes: #305