BUILD: args: use __fallthrough in make_arg_list()

This avoids one build warning when preprocessing happens before compiling
with gcc >= 7.
diff --git a/src/arg.c b/src/arg.c
index 039602a..17f5ed0 100644
--- a/src/arg.c
+++ b/src/arg.c
@@ -250,8 +250,8 @@
 				goto resolve_err;
 			arg->unresolved = 1;
 			new_al = arg_list_add(al, arg, pos);
+			__fallthrough;
 
-			/* fall through */
 		case ARGT_STR:
 			/* all types that must be resolved are stored as strings
 			 * during the parsing. The caller must at one point resolve