perf build: Clarify header version warning message

Change this:

  Warning: arch/x86/include/asm/disabled-features.h differs from kernel
  Warning: arch/x86/include/asm/cpufeatures.h differs from kernel
  Warning: arch/powerpc/include/uapi/asm/kvm.h differs from kernel
  Warning: arch/s390/include/uapi/asm/kvm.h differs from kernel

  Warning: Kernel ABI header at 'tools/arch/x86/include/asm/disabled-features.h' differs from latest version at 'arch/x86/include/asm/disabled-features.h'
  Warning: Kernel ABI header at 'tools/arch/x86/include/asm/cpufeatures.h' differs from latest version at 'arch/x86/include/asm/cpufeatures.h'
  Warning: Kernel ABI header at 'tools/arch/powerpc/include/uapi/asm/kvm.h' differs from latest version at 'arch/powerpc/include/uapi/asm/kvm.h'
  Warning: Kernel ABI header at 'tools/arch/s390/include/uapi/asm/kvm.h' differs from latest version at 'arch/s390/include/uapi/asm/kvm.h'

... to make it clearer what the warning is about, and to make it easier
to diff the two versions when syncing up the files.
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: David Carrillo-Cisneros <davidcc@google.com>
Cc: Francis Deslauriers <francis.deslauriers@efficios.com>
Cc: Geneviève Bastien <gbastien@versatic.net>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Julien Desfossez <jdesfossez@efficios.com>
Cc: Martin Liška <mliska@suse.cz>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Turner <pjt@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Simon Que <sque@chromium.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Taeung Song <treeze.taeung@gmail.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20170730093747.qogjn3lp7ntwcgwg@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/check-headers.sh

@@ -47,7 +47,7 @@ check () {
   cmd="diff $opts ../$file ../../$file > /dev/null"
 
   test -f ../../$file &&
-  eval $cmd || echo "Warning: $file differs from kernel" >&2
+  eval $cmd || echo "Warning: Kernel ABI header at 'tools/$file' differs from latest version at '$file'" >&2
 }