libhif.pc.in: point Cflags to the includedir only
libhif.pc.in: point Cflags to the includedir only

We would previously set Cflags to include the libhif directory itself.
However, this causes problems for library users since libhif headers
themselves use include directives with the libhif prefix.

Let's change the Cflags to point to just ${includedir} and require users
to always prefix their includes.

Closes: #138
Approved by: cgwalters

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