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

arch/powerpc/platforms/pseries/vas.c

@@ -184,7 +184,7 @@ static int h_get_nx_fault(u32 winid, u64 buffer)
  * Note: The hypervisor forwards an interrupt for each fault request.
  *	So one fault CRB to process for each H_GET_NX_FAULT hcall.
  */
-irqreturn_t pseries_vas_fault_thread_fn(int irq, void *data)
+static irqreturn_t pseries_vas_fault_thread_fn(int irq, void *data)
 {
 	struct pseries_vas_window *txwin = data;
 	struct coprocessor_request_block crb;