Make libbrillo-dbus compilation optional based on dbus USE flag.

On branches where DBus is not available we need to disable the
compilation of all the targets requiring DBus.

Bug: 25197634
Test: `mmma external/libbrillo` with the BRILLO_USE_DBUS := 0 in the .mk
Test: FEATURES=test emerge-link libbrillo

Change-Id: I4924c93e65ef14c732eea6ee7a73beb118538585
2 files changed