Add check for LLC_OPT_PKTINFO for older distributions.

This patch fixes this build error on distributions, such as CentOS 6 and
Ubuntu 10.04, that have older kernels.

  CC    syscalls/setsockopt.o
In file included from syscalls/setsockopt.c:13:
./syscalls/setsockopt.h:167: error: ‘LLC_OPT_PKTINFO’ undeclared here (not in a function)
make: *** [syscalls/setsockopt.o] Error 1

Tested-by: Vinson Lee <vlee@twitter.com>