perf symbols: Add nr_events to symbol_conf

Since symbol__alloc_hists need it, to avoid passing it around in many
functions have it in the symbol_conf struct.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
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-cwv8ysvpywzjq4v3xtbd4zwv@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>