plugin: plug memleak

A new reference to probe was acquired through mm_device_get_probe(), just unref it
when no longer needed. Note that mm_port_probe_run() will take its own reference
for as long as required to complete the operation.

src/mm-plugin.c

@@ -750,6 +750,7 @@ mm_plugin_supports_port (MMPlugin *self,
 
 out:
     g_object_unref (async_result);
+    g_object_unref (probe);
 }
 
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