[docs] add dependency when building dbus docs (#1118)
[docs] add dependency when building dbus docs (#1118)

The `otbr-dbus-server-doc-copy` target depends on the output of the
Doxygen build. Add a dependency on the Doxgyen build.
openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter