tool/make_dbus: use __attribute__((weak)) declare dbus callback

one project only support one dbus.go configure. but apps like
greeter has multi binaries program. If the file which defined
dbus callback didn't linked with the binary then there will be a
compile error.

Use __attribute__((weak)) to handle this.

Change-Id: Ic62c36747118b0c03ea911fcadc1c95303155b56