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.

file.c

@@ -1021,6 +1021,7 @@ idprintf(FILEID id, char *fmt, int count, VALUE **vals)
 		switch (ch) {
 		case 's':
 			printstring = TRUE;
+            /* fall through */
 		case 'c':
 			printchar = TRUE;
 		case 'd':