x86: Compile insn.c and inat.c only for KPROBES

At least, insn.c and inat.c is needed for kprobe for now. So,
this compile those only if KPROBES is enabled.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Masami Hiramatsu <mhiramat@redhat.com>
LKML-Reference: <878wdg8icq.fsf@devron.myhome.or.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>