Fix: older kernels (3.2.x) don't undefine TRACE_SYSTEM

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Fix instrumentation of vmscan for older kernels

Missing definition of LTTNG_TRACEPOINT_EVENT_CONDITION_MAP and
LTTNG_TRACEPOINT_EVENT_INSTANCE_CONDITION_MAP, only used for rather old
kernels (e.g. 3.2.62).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>