MIPS: RB532: Fix build error

mips/pci/fixup-rc32434.c must #include <asm/mach-rc32434/irq.h>

This patch fixes the following compile error caused by
commit 606a083b
(MIPS: RB532: Cleanup the headers again):

<--  snip  -->

...
  CC      arch/mips/pci/fixup-rc32434.o
arch/mips/pci/fixup-rc32434.c: In function 'pcibios_map_irq':
arch/mips/pci/fixup-rc32434.c:46: error: 'GROUP4_IRQ_BASE' undeclared (first use in this function)
arch/mips/pci/fixup-rc32434.c:46: error: (Each undeclared identifier is reported only once
arch/mips/pci/fixup-rc32434.c:46: error: for each function it appears in.)
make[2]: *** [arch/mips/pci/fixup-rc32434.o] Error 1

<--  snip  -->

A virtually identical patch was also submitted by Yoichi Yuasa
<yoichi_yuasa@tripeaks.co.jp>.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>