markers: fix synchronize marker unregister static inline

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: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>