powerpc/pseries/vas: Declare pseries_vas_fault_thread_fn() as static

This fixes a compile error with W=1.

Fixes: 6d0aaf5e ("powerpc/pseries/vas: Setup IRQ and fault handling")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210819125656.14498-3-clg@kaod.org