tools/insn: Restore the relative include paths for cross building

Building perf on ppc causes:

  In file included from util/intel-pt-decoder/intel-pt-insn-decoder.c:15:
  util/intel-pt-decoder/../../../arch/x86/lib/insn.c:14:10: fatal error: asm/inat.h: No such file or directory
     14 | #include <asm/inat.h> /*__ignore_sync_check__ */
        |          ^~~~~~~~~~~~

Restore the relative include paths so that the compiler can find the
headers.

Fixes: 93281c4a ("x86/insn: Add an insn_decode() API")
Reported-by: Ian Rogers <irogers@google.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Borislav Petkov <bp@suse.de>
Tested-by: Ian Rogers <irogers@google.com>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lkml.kernel.org/r/20210317150858.02b1bbc8@canb.auug.org.au