vsock: fix `vsock_proto` declaration

vsock: fix `vsock_proto` declaration

From commit 634f1a7110b4 ("vsock: support sockmap"), `struct proto
vsock_proto`, defined in af_vsock.c, is not static anymore, since it's
used by vsock_bpf.c.

If CONFIG_BPF_SYSCALL is not defined, `make C=2` will print a warning:
    $ make O=build C=2 W=1 net/vmw_vsock/
      ...
      CC [M]  net/vmw_vsock/af_vsock.o
      CHECK   ../net/vmw_vsock/af_vsock.c
    ../net/vmw_vsock/af_vsock.c:123:14: warning: symbol 'vsock_proto' was not declared. Should it be static?

Declare `vsock_proto` regardless of CONFIG_BPF_SYSCALL, since it's defined
in af_vsock.c, which is built regardless of CONFIG_BPF_SYSCALL.

Fixes: 634f1a7110b4 ("vsock: support sockmap")
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://patch.msgid.link/20250703112329.28365-1-sgarzare@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>