NO-OP: fix some Wimplicit-fallthrough

gcc can recognize various regexps in comments. Since C++17 provides
[[fallthrough]], using /* fallthrough */ consistently seems
appropriate until we switch to C++17.

see also https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

session_utils/export.cc

@@ -274,7 +274,7 @@ int main (int argc, char* argv[])
 							settings._sample_format = ExportFormatBase::SF_Float;
 							break;
 						}
-						/* fall through */
+						/* fallthrough */
 					default:
 						fprintf(stderr, "Invalid Bit Depth\n");
 						break;