[POWERPC] Move xmon_irq() declaration into xmon.h

The typdef for irqreturn_t was moved into its own header a
while back, so there's no reason we can't move xmon_irq()
into xmon.h now.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>