message: Fix gpuvis_trace_utils.h struct has no members warnings
  struct GpuvisTraceBlock {};
  src/../sample/gpuvis_trace_utils.h:287:8: warning:
  struct has no members [-Wpedantic]
diff: @@ -279,12 +279,11 @@ static inline int gpuvis_tracing_on() { return -1; }
  static inline const char *gpuvis_get_tracefs_dir() { return ""; }
  static inline const char *gpuvis_get_tracefs_filename( char *buf, size_t buflen, const char *file ) { return NULL; }
  -struct GpuvisTraceBlock {};
  -
  +struct GpuvisTraceBlock;
  static inline void gpuvis_trace_block_begin( struct GpuvisTraceBlock *block, const char *str ) {}
  static inline void gpuvis_trace_block_end( struct GpuvisTraceBlock *block ) {}
  -struct GpuvisTraceBlockf {};
  +struct GpuvisTraceBlockf;
  static inline void gpuvis_trace_blockf_vbegin( struct GpuvisTraceBlockf *block, const char *fmt, va_list ap ) {}
  static inline void gpuvis_trace_blockf_begin( struct GpuvisTraceBlockf *block, const char *fmt, ... ) {}
  static inline void gpuvis_trace_blockf_end( struct GpuvisTraceBlockf *block ) {}
  