bpfilter: fix OUTPUT_FORMAT

CONFIG_OUTPUT_FORMAT is x86 only macro.
Used objdump to extract elf file format.

Fixes: d2ba09c1

 ("net: add skeleton of bpfilter kernel module")
Reported-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>