knl_cray plugin - increase output buffer to silence warning.

GCC-7 with -Wformat warns about

node_features_knl_cray.c:2869:33: warning: ‘%d’ directive output may be
truncated writing between 1 and 10 bytes into a region of size 8
[-Wformat-truncation=]
     snprintf(buf, sizeof(buf), "%d", i);
                                 ^~
node_features_knl_cray.c:2869:32: note: directive argument in the
range [0, 2147483647]
     snprintf(buf, sizeof(buf), "%d", i);
                                ^~~~
In file included from /usr/include/stdio.h:862:0,
                 from ../../../../slurm/slurm.h:68,
                 from node_features_knl_cray.c:66:
/usr/include/bits/stdio2.h:64:10: note: ‘__builtin___snprintf_chk’
output between 2 and 11 bytes into a destination of size 8
   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        __bos (__s), __fmt, __va_arg_pack ());
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Increasing the buffer to 12 for this.

Bug 4900.

Signed-off-by: Justin Lecher <jlec@gentoo.org>