Revert "kernel: kcov: include missing header for ‘struct task_struct’"

This reverts commit 79becfb7.

The kcov feature is being reverted, so we don't need this spot fix any
more.

Bug: 116647072
Change-Id: I9391d75afa456d5a7070b13d0e1657f4cfc02a22
Signed-off-by: Alistair Strachan <astrachan@google.com>