[POWERPC] Fix compile error in prom.h

In file included from include/asm/pci.h:20,
                 from include/linux/pci.h:751,
                 from arch/powerpc/sysdev/dart_iommu.c:36:
include/asm/prom.h: In function `of_irq_to_resource':
include/asm/prom.h:341: warning: implicit declaration of function `irq_of_parse_and_map'
include/asm/prom.h:345: error: `NO_IRQ' undeclared (first use in this function)
include/asm/prom.h:345: error: (Each undeclared identifier is reported only once
include/asm/prom.h:345: error: for each function it appears in.)

Seems that prom.h has always wanted irq.h.

Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>