htfollow.cc: include <cstdio> for snprintf
htfollow.cc: include <cstdio> for snprintf

Change htfollow.cc to include the <cstdio> header for getting the
snprintf function.

This squashes the following error, as reported by clang++(1):

    htfollow.cc:28:3: error: use of undeclared identifier 'snprintf'
      snprintf(buf, sizeof(buf), "%d", i);
      ^
    1 diagnostic generated.

Signed-off-by: Ævar Arnfjörð Bjarmason <0135fcc047f75af264c800977a73e353c6ba842d@gmail.com>

avar/fIcy