perf symbols: Fix reading of build-id from vDSO

We need to use the long name (the filename) when reading the build-id
from a DSO.  Using the short name doesn't work for (at least) vDSOs.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20160113172301.GT28542@decadent.org.uk


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