Skips changing prosody config on upgrading jitsi-meet-tokens package.

debian/jitsi-meet-tokens.postinst

         db_stop
 
         if [ -f "$PROSODY_HOST_CONFIG" ] ; then
-            if grep -q "plugin_paths" "$PROSODY_HOST_CONFIG"; then
+            # search for --plugin_paths, if this is not enabled this is the
+            # first time we install tokens package and needs a config change
+            if grep -q "\-\-plugin_paths" "$PROSODY_HOST_CONFIG"; then
                 # enable tokens in prosody host config
                 sed -i 's/--plugin_paths/plugin_paths/g' $PROSODY_HOST_CONFIG
                 sed -i 's/authentication = "anonymous"/authentication = "token"/g' $PROSODY_HOST_CONFIG