dumpscan: appease gcc8 -Wformat-overflow

gcc does not benefit from our external knowledge that tm_year is
tightly bounded, and thinks it could still be in the range
[-2147481748, 2147483647], which would overflow our string buffer.
The function in question does not have error handling in place, so
rather than adding some or trying to assert the proper bounds, just
use a slightly larger buffer for safety.

Change-Id: Iafcba5588b805347ddcc0102969bd0e2a3173dd0
Reviewed-on: https://gerrit.openafs.org/13472
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>

src/tools/dumpscan/afsdump_extract.c

@@ -237,7 +237,7 @@ static char *month[] = { "Jan", "Feb", "Mar", "Apr", "May", "Jun",
 static char *
 datestr(time_t date)
 {
-    static char str[13];
+    static char str[20];
     time_t clock = time(0);
     struct tm *now, *then;
     int diff;