Building latest perfcounter fails on the following error:

 builtin-record.c: In function ‘create_counter’:
 builtin-record.c:451: erreur: ignoring return value of ‘read’, declared with attribute warn_unused_result
 make: *** [builtin-record.o] Erreur 1

Just check if we successfully read the perf file descriptor.
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <1245961287-5327-1-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>