markers: add missing stdargs.h include, needed due to va_list usage

Impact: build fix (for future changes)

That seemed to cause built issue when marker.h is included early, even
though stdargs.h is included in kernel.h.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Ingo Molnar <mingo@elte.hu>