perf tools: Fix GNU-only grep usage in Makefile

This makes it work with non-GNU grep's as well.

Signed-off-by: John Spencer <maillist-linux@barfooze.de>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: http://thread.gmane.org/gmane.linux.kernel.perf.user/1686


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>