perf tools: Do not check ABI headers in a detached tarball build

When we use one of:

  [acme@jouet linux]$ make help | grep perf
    perf-tar-src-pkg    - Build perf-4.14.0-rc3.tar source tarball
    perf-targz-src-pkg  - Build perf-4.14.0-rc3.tar.gz source tarball
    perf-tarbz2-src-pkg - Build perf-4.14.0-rc3.tar.bz2 source tarball
    perf-tarxz-src-pkg  - Build perf-4.14.0-rc3.tar.xz source tarball
  [acme@jouet linux]$

I.e. when we create a detached tarball to build perf outside outside the
enveloping kernel sources (from a kernel tarball or a checked out
linux.git directory) we by definition can't check for differences among
the tools/{include,arch}, etc files we originally copied from the
kernel, so bail out in that case, to avoid warnings when doing the
detached builds.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-vbrga0mhplv7niwxr3ghjyxv@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>