ARM: fix build warning in asm/elf.h

  CC      kernel/elfcore.o
In file included from include/linux/elf.h:7,
                 from kernel/elfcore.c:1:
arch/arm/include/asm/elf.h:124: warning: 'struct mm_struct' declared inside parameter list
arch/arm/include/asm/elf.h:124: warning: its scope is only this definition or declaration, which is probably not what you want

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>