perf symbols: Constify some DSO methods parameters

Those methods are not supposed to change the data structures they
manipulate, so make that clearer by using the const qualifier in the
function signature and in some variables.

Suggested-by: Ingo Molnar <mingo@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-j7oyakex7zy3r82h33rdw25x@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>