net: sctp: attribute printl with __printf for gcc fmt checks

Let GCC check for format string errors in sctp's probe printl
function. This patch fixes the warning when compiled with W=1:

net/sctp/probe.c:73:2: warning: function might be possible candidate
for 'gnu_printf' format attribute [-Wmissing-format-attribute]

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>