message: [docs] add dependency when building dbus docs
  The `otbr-dbus-server-doc-copy` target depends on the output of the
  Doxygen build. Add a dependency on the Doxgyen build.
diff: @@ -51,7 +51,7 @@ if (DOXYGEN_FOUND)
  COMMAND cp ${DBUS_DOC_SRC} ${DBUS_DOC_TARGET}
  VERBATIM
  )
  - add_dependencies(otbr-dbus-server-doc-copy otbr-dbus-server-doc)
  + add_dependencies(otbr-dbus-server-doc-copy doxygen otbr-dbus-server-doc)
  add_dependencies(otbr-doc otbr-dbus-server-doc-copy)
  endif()
  else()
  