If we're linking to libdbus-1, qdbus_symbols_p.h #includes dbus/dbus.h,
which may be old and not #define the Unix FD macros. tst_QDBusMarshall
had the definition for one of the macros but not the other, so add the
missing second one.

Change-Id: Id69569111e7d4e619e22ffff144c35edcd6f348b
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>