timers: Remove stale struct tvec_base forward declaration

struct tvec_base is a leftover of the original timer wheel implementation
and not longer used. Remove the forward declaration.

Signed-off-by: Liu Changcheng <changcheng.liu@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: akpm@linux-foundation.org
Link: https://lkml.kernel.org/r/20180412075701.GA38952@sofia