shared-qmi: fix memory leak in set_supl_server command

ctx->supl is allocated in mm_shared_qmi_location_set_supl_server
using g_strdup. g_strdup requires to free string when unneeded.

The problem could be reproduced using following commands
```
 while true; do
       mmcli -m any --location-set-supl-server="supl.google.com:7276";
       mem=$(cat /proc/$(pgrep ModemManager)/statm | awk '{print $6}')
       echo $(date +"%s"), $mem | tee -a mm-leak.log;
 done;
```

src/mm-shared-qmi.c

@@ -4464,6 +4464,7 @@ set_supl_server_context_free (SetSuplServerContext *ctx)
             g_signal_handler_disconnect (ctx->client, ctx->indication_id);
         g_object_unref (ctx->client);
     }
+    g_free (ctx->supl);
     g_slice_free (SetSuplServerContext, ctx);
 }