security: bpf: replace include of linux/bpf.h with forward declarations

Touching linux/bpf.h makes us rebuild a surprisingly large
portion of the kernel.  Remove the unnecessary dependency
from security.h, it only needs forward declarations.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>