UML: Fix compiler warning due to missing task_struct declaration

We can't pull in linux/sched.h due to circular dependency, so just
forward-declare the struct.

This fixes the following warning:

  CC      arch/um/sys-i386/elfcore.o
In file included from /data/linux-2.6/include/linux/elf.h:8,
                 from /data/linux-2.6/arch/um/sys-i386/elfcore.c:2:
arch/um/sys-i386/asm/elf.h:78: warning: ‘struct task_struct’ declared inside parameter list
arch/um/sys-i386/asm/elf.h:78: warning: its scope is only this definition or declaration, which is probably not what you want

I guess not many people build against i386 hosts anymore, so this
remained widely unnoticed.

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>