powerpc/tau: Add 'static' storage qualifier to 'tau_work' definition

This patch prevents the following sparse warning.

arch/powerpc/kernel/tau_6xx.c:199:1: sparse: sparse: symbol 'tau_work'
was not declared. Should it be static?
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/44ab381741916a51e783c4a50d0b186abdd8f280.1629334014.git.fthain@linux-m68k.org

arch/powerpc/kernel/tau_6xx.c

@@ -164,7 +164,7 @@ static void tau_work_func(struct work_struct *work)
 	queue_work(tau_workq, work);
 }
 
-DECLARE_WORK(tau_work, tau_work_func);
+static DECLARE_WORK(tau_work, tau_work_func);
 
 /*
  * setup the TAU