Fix: make writeback instrumentation build for 4.3 kernel

The backported patch "Fix: writeback instrumentation update for 4.3
Linux kernel" (ee33121c8ff05b44ab) used a newer definition of
LTTNG_TRACEPOINT_EVENT_CLASS which is not available in v2.6.

Note that, while this fixes the build, the global_dirty_limit symbol
used by some writeback events is no longer present since the 4.2 kernel,
and will result in runtime "lookup failed" warnings if those events are
used.  This is also the case on current master.

Signed-off-by: Rabin Vincent <rabinv@axis.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>