posix-timers: Cleanup forward declarations and includes

 - Rename struct siginfo to kernel_siginfo as that is used and required
 - Add a forward declaration for task_struct and remove sched.h include
 - Remove timex.h include as it is not needed

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lkml.kernel.org/r/20190819143801.472005793@linutronix.de