add forward declaration of bpf_insn in bpf_module.h (#2269)
add forward declaration of bpf_insn in bpf_module.h (#2269)

struct bpf_insn is used (as a pointee) in bpf_module.h,
but no definition or forward declaration can be found
by just looking at this file.

definition is not needed as it is used as a pointee.
providing a forward declaration here.

Signed-off-by: Yonghong Song <e76892bbe783db2f627de1232e139db4de43da60@fb.com>
tuxology/bcc,brendangregg/bcc,tuxology/bcc,iovisor/bcc,iovisor/bcc,brendangregg/bcc,brendangregg/bcc,brendangregg/bcc,iovisor/bcc,iovisor/bcc,tuxology/bcc,brendangregg/bcc,tuxology/bcc,tuxology/bcc,iovisor/bcc