perf trace: Deal with compiler const checks

The strchr() function these days return const/non-const based on the arg
it receives, and sometimes we need to use casts when we're dealing with
variables that are used in code that needs to safely change the returned
value and sometimes not (as it points to really const areas).

Tweak one such case.

Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>