ARM: 5987/1: fix warning in kernel/elfcore.c from ARM's elf.h

ARM: 5987/1: fix warning in kernel/elfcore.c from ARM's elf.h

2.6.34-rc1 added kernel/elfcore.c which includes <asm/elf.h>.
On ARM, this results in:

In file included from include/linux/elf.h:7,
                 from kernel/elfcore.c:1:
/tmp/linux-2.6.34-rc1/arch/arm/include/asm/elf.h:101: warning: 'struct task_struct' declared inside parameter list
/tmp/linux-2.6.34-rc1/arch/arm/include/asm/elf.h:101: warning: its scope is only this definition or declaration, which is probably not what you want

Including <linux/sched.h> seems a bit heavyweight, so this patch just
adds a tentative declaration of struct task_struct in <asm/elf.h>.

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>