Use a #define for synchronize marker unregister to fix include dependencies.

Fixes the slab circular inclusion which triggers when slab.git is combined
with tracing.git, where rcupdate includes slab, which includes markers
which includes rcupdate.
Signed-off-by: NMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Acked-by: NPekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: NIngo Molnar <mingo@elte.hu>