perf build-id: Fix filename size calculation

The filename is a pointer variable so the sizeof(filename) will return
length of a pointer. Fix it by using 'size'.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1335881976-3282-1-git-send-email-namhyung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>