Fix build on kernels without perf_event_attr exclude_callchain_kernel.

This patch fixes these build errors on 3.6 and older.

  CC	syscalls/perf_event_open.o
syscalls/perf_event_open.c: In function ‘create_mostly_valid_counting_event’:
syscalls/perf_event_open.c:382:6: error: ‘struct perf_event_attr’ has no member named ‘exclude_callchain_kernel’
syscalls/perf_event_open.c:383:6: error: ‘struct perf_event_attr’ has no member named ‘exclude_callchain_user’
syscalls/perf_event_open.c: In function ‘create_mostly_valid_sampling_event’:
syscalls/perf_event_open.c:431:6: error: ‘struct perf_event_attr’ has no member named ‘exclude_callchain_kernel’
syscalls/perf_event_open.c:432:6: error: ‘struct perf_event_attr’ has no member named ‘exclude_callchain_user’
make: *** [syscalls/perf_event_open.o] Error 1

Signed-off-by: Vinson Lee <vlee@twitter.com>