libhif.pc: Fix prefix variables for cmake
libhif.pc: Fix prefix variables for cmake

We were installing prefix=/.  I stole these bits from librepo.

Conan-Kudo/libhif,Conan-Kudo/libhif,rpm-software-management/libdnf,edynox/libdnf,rpm-software-management/libhif,hughsie/libhif,rpm-software-management/libdnf,rpm-software-management/libdnf,Conan-Kudo/libhif,hughsie/libhif,hughsie/libhif,Conan-Kudo/libhif,rpm-software-management/libhif,edynox/libdnf,rpm-software-management/libhif,edynox/libdnf,rpm-software-management/libhif,edynox/libdnf,hughsie/libhif,rpm-software-management/libdnf