build: Install hif-enums.h
build: Install hif-enums.h

This is a regression from 72cf2bce90931fe6a5f4aa43901480a89c6c282d
where the header was introduced, and other public headers
depend on it.

Without this, dependent C projects like rpm-ostree will fail to build.
Closes: #108

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