perf script: Fix symbol insertion behavior in db-export

perf script: Fix symbol insertion behavior in db-export

Use the dso__insert_symbol function instead of symbols__insert() in
order to properly update the dso symbol cache.

If the cache is not updated, then duplicate symbols can be
unintentionally created, inserted, and exported.

This change prevents duplicate symbols from being exported due to
dso__find_symbol() using a stale symbol cache.

Signed-off-by: Chris Phlipot <cphlipot0@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1462937209-6032-3-git-send-email-cphlipot0@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>