BUG/MINOR: task: fix missing include with DEBUG_TASK

Following include reorganzation, there is some missing include files for
task.h when compiling with DEBUG_TASK :
- activity.h for task_profiling_mask
- time.h for now_mono_time()

This is present since the following commit
  d8b325c74826bdb02759f62c41a00455dbae3431
  REORG: task: uninline the loop time measurement code

No need to backport this.
1 file changed