bpf: Emit struct bpf_xdp_sock type in vmlinux BTF

Similar to other BPF UAPI struct, force emit BTF of struct bpf_xdp_sock
so that it is defined in vmlinux.h.

In a later patch, a selftest will use vmlinux.h to get the definition of
struct bpf_xdp_sock instead of bpf.h.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250925170013.1752561-1-ameryhung@gmail.com