perf tools: Make check-headers.sh check based on kernel dir

Changing the logic to compare files with paths relative to kernel source
base dir. This way we can keep the output message for 2 unrelated files,
which is coming in following patch.

Committer testing:

Remove a line from tools/arch/x86/lib/memcpy_64.S to have it detected:

make: Entering directory '/home/acme/git/perf/tools/perf'
  BUILD:   Doing 'make -j4' parallel build
Warning: Kernel ABI header at 'tools/arch/x86/lib/memcpy_64.S' differs from latest version at 'arch/x86/lib/memcpy_64.S'
diff -u tools/arch/x86/lib/memcpy_64.S arch/x86/lib/memcpy_64.S
  INSTALL  GTK UI
  INSTALL  binaries

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Kapshuk <alexander.kapshuk@gmail.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180813111504.3568-1-jolsa@kernel.org
Link: http://lkml.kernel.org/r/20180814072726.GA13931@krava


[ Do not use pushd/popd, its a bashism, reported by Michael Ellerman, fixed by Jiri Olsa ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>