perf_events, x86: Fix event constraints code

There was namespace overlap due to a rename i did - this caused
the following build warning, reported by Stephen Rothwell against
linux-next x86_64 allmodconfig:

  arch/x86/kernel/cpu/perf_event.c: In function 'intel_get_event_idx':
  arch/x86/kernel/cpu/perf_event.c:1445: warning: 'event_constraint' is used uninitialized in this function

This is a real bug not just a warning: fix it by renaming the
global event-constraints table pointer to 'event_constraints'.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Stephane Eranian <eranian@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <20091013144223.369d616d.sfr@canb.auug.org.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>