exhtdr: pacify compiler warning

exhtdr: pacify compiler warning

gcc 7.1.1 complains:

exthdr.c:41:31: warning: ‘%d’ directive output may be truncated writing between 1 and 8 bytes into a region of size 3 [-Wformat-truncation=]
    snprintf(buf, sizeof buf, "%d", offset);
                               ^~
This warning is incorrect, as offset is limited by tcp option ranges,
but gcc doesn't know this. Increase buffer to avoid the warning.

Signed-off-by: Florian Westphal <fw@strlen.de>