firewalld: Build own DBus proxies.

This will cause DBus proxies to be generated, along with corresponding
pkg_config files. An upcoming ebuild change will actually install them
to the sysroot.

[Copied over from https://chromium-review.googlesource.com/293616/]

Bug: 22827985
Change-Id: I4a5140b985d73a817e36c26b38871ef8b114288d
4 files changed