Shill requires librt for clock_gettime

It used to mooch this dependency off of dbus, but the new
dbus no longer links against it.  Specify this dependency
in the Makefile, as it should be.

BUG=chromium-os:39738
TEST=FEATURES=test emerge-amd64-generic shill

Change-Id: Ic6aabd210d05b47b4cea69687f36414207cc09a7
Reviewed-on: https://gerrit.chromium.org/gerrit/45028
Tested-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Chris Masone <cmasone@chromium.org>