bpf: move bpf_jit_enable declaration

Move the bpf_jit_enable declaration to the filter.h file where
most other core code is declared, also since we're going to add
a second knob there.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>