backend: Fix GProxyResolver warnings

src/backend/bacon-video-widget.c: In function ‘bvw_set_http_proxy_on_element’:
src/backend/bacon-video-widget.c:2654:8: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   host = gst_uri_get_host (uri);
        ^
src/backend/bacon-video-widget.c:2657:13: warning: assignment to ‘char’ from ‘gchar *’ {aka ‘char *’} makes integer from pointer without a cast [-Wint-conversion]
   proxy_url = g_strdup_printf ("%s://%s:%d", protocol, host, port);
             ^
src/backend/bacon-video-widget.c:2659:11: warning: passing argument 1 of ‘g_free’ makes pointer from integer without a cast [-Wint-conversion]
   g_free (proxy_url);
           ^~~~~~~~~
In file included from src/backend/bacon-video-widget.c:49:
/home/hadess/Projects/gnome-install/include/glib-2.0/glib/gmem.h:71:35: note: expected ‘gpointer’ {aka ‘void *’} but argument is of type ‘char’
 void  g_free           (gpointer  mem);
                         ~~~~~~~~~~^~~
src/backend/bacon-video-widget.c:2665:12: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   userinfo = gst_uri_get_userinfo (uri);
            ^

Fixes: 12aa8c7b