base/trace/buffer.h: fix circular #include

Previously, base/trace/buffer.h included base/thread.h which includes
base/trace/logger.h which includes base/trace/buffer.h.

Removed the base/thread.h include in base/trace/buffer.h as it is not
needed.

Issue #2654

repos/base/include/base/trace/buffer.h

@@ -15,7 +15,6 @@
 #define _INCLUDE__BASE__TRACE__BUFFER_H_
 
 #include <base/stdint.h>
-#include <base/thread.h>
 #include <cpu_session/cpu_session.h>
 
 namespace Genode { namespace Trace { class Buffer; } }