um: Enforce the usage of asm-generic/softirq_stack.h

The recent rework of the X86 irq stack switching mechanism broke UM as UM
pulls in the X86 specific variant of softirq_stack.h.

Enforce the usage of the asm-generic variant.

Fixes: 72f40a28 ("x86/softirq/64: Inline do_softirq_own_stack()")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Richard Weinberger <richard@nod.at>

arch/um/include/asm/Kbuild

@@ -21,6 +21,7 @@ generic-y += param.h
 generic-y += pci.h
 generic-y += percpu.h
 generic-y += preempt.h
+generic-y += softirq_stack.h
 generic-y += switch_to.h
 generic-y += topology.h
 generic-y += trace_clock.h