add comments to suppres -Wimplicit-fallthrough warnings

Origin commit data
------------------
Commit: 971f3703a6
Author: Stefan Seyfried <seife@tuxbox-git.slipkontur.de>
Date: 2017-09-23 (Sat, 23 Sep 2017)

src/neutrino.cpp

@@ -1790,6 +1790,7 @@ void CNeutrinoApp::SetChannelMode(int newmode)
 			break;
 		default:
 			newmode = LIST_MODE_PROV;
+			/* fall through */
 		case LIST_MODE_PROV:
 			if(mode == mode_radio)
 				bouquetList = RADIObouquetList;