perf tp_pmu: Address const-correctness errors in recent glibcs

perf tp_pmu: Address const-correctness errors in recent glibcs

To avoid having more variables, just cast the const variable searched to
non-const since the result will not be modified, its only later that
that variable will be used to modify something, but then its non-const
memory being modified, so using a cast is the cheapest thing here.

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