Comment fallthrough case statements so compilation succeeds with GCC 7.

GCC 7 Added a warning for implicit fallthroughs in switch cases.
It's enabled by -Wextra, and treated as an error due to -Werror so
compilation fails on GCC 7 and higher.
See -Wimplicit-fallthrough in the GCC manual.

qio.c

@@ -144,6 +144,7 @@ qprintf(char *fmt, ...)
 		case '-':
 			sign = -1;
 			ch = *fmt++;
+            /* fall through */
 		default:
 			if (('0' <= ch && ch <= '9') ||
 			    ch == '.' || ch == '*') {