perf tools: Version incorrect with some versions of grep

perf tools: Version incorrect with some versions of grep

Some versions of grep don't treat '\s' properly. When building perf on such
systems and using a kernel tarball the perf version is unable to be determined
from the main kernel Makefile and the user is left with a version of '..'.
Replacing the use of '\s' with '[[:space:]]', which should work in all grep
versions, gives a usable version number.

Reported-by: Tapan Dhimant <tdhimant@akamai.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Tapan Dhimant <tdhimant@akamai.com>
Cc: linux-kernel@vger.kernel.org
Cc: stable@kernel.org
LKML-Reference: <1300241800-30281-1-git-send-email-johunt@akamai.com>
Signed-off-by: Josh Hunt <johunt@akamai.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>