um: irq.h: include <asm-generic/irq.h>

This will get the (no-op) definition of irq_canonicalize()
which some code might want. We could define that ourselves,
but it seems like we'd likely want generic extensions in
the future, if any.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>