perf_counter: fix powerpc build

Commit 4af4998b ("perf_counter: rework context time") changed struct
perf_counter_context to have a 'time' field instead of a 'time_now'
field, but neglected to fix the place in the powerpc perf_counter.c
where the time_now field was accessed.  This fixes it.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <18908.31922.411398.147810@cargo.ozlabs.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>