selftests/bpf: Set CONFIG_PACKET=y for selftests

selftests/bpf: Set CONFIG_PACKET=y for selftests

BPF selftest fails to build with below error:

  CLNG-BPF [test_progs] lsm_cgroup.bpf.o
progs/lsm_cgroup.c:105:21: error: variable has incomplete type 'struct sockaddr_ll'
  105 |         struct sockaddr_ll sa = {};
      |                            ^
progs/lsm_cgroup.c:105:9: note: forward declaration of 'struct sockaddr_ll'
  105 |         struct sockaddr_ll sa = {};
      |                ^
1 error generated.

lsm_cgroup selftest requires sockaddr_ll structure which is not there
in vmlinux.h when the kernel is built with CONFIG_PACKET=m.

Enabling CONFIG_PACKET=y ensures that sockaddr_ll is available in vmlinux,
allowing it to be captured in the generated vmlinux.h for bpf selftests.

Reported-by: Sachin P Bappalige <sachinpb@linux.ibm.com>
Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20250707071735.705137-1-skb99@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>