Increase mutt_pgpring path size to silence warning.

This is an independent binary, and doesn't have access to the BUFFER
functions. The binary is used by very few (if any) people at this
point, so just make the minimum fix to silence the warning.

pgppubring.c

···
   char *env_pgppath, *env_home;
 
   char pgppath[_POSIX_PATH_MAX];
-  char kring[_POSIX_PATH_MAX];
+  char kring[_POSIX_PATH_MAX+12];
 
   while ((c = getopt (argc, argv, "f25sk:S")) != EOF)
   {