live555: add "pnm" alias as used for Real streams

radio-france.fr seems to use such URLs
example: http://son.radio-france.fr/chaines/itema/10491/10491_02042010_2010F10491S0092.rm

All hail to our new owners !

modules/demux/live555.cpp

@@ -107,6 +107,7 @@ vlc_module_begin ()
     add_submodule ()
         set_description( N_("RTSP/RTP access and demux") )
         add_shortcut( "rtsp" )
+        add_shortcut( "pnm" )
         add_shortcut( "sdp" )
         add_shortcut( "live" )
         add_shortcut( "livedotcom" )