perf probe: Make a copy of exec path for passing to basename

The basename function may modify the string passed to it, so the string
should not be marked const.

Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Pekka Enberg <penberg@kernel.org>
Cc: Irina Tirdea <irina.tirdea@gmail.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1347116812-93646-3-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>