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>

arch/arm/include/asm/elf.h

@@ -121,6 +121,7 @@ int dump_task_regs(struct task_struct *t, elf_gregset_t *elfregs);
 extern void elf_set_personality(const struct elf32_hdr *);
 #define SET_PERSONALITY(ex)	elf_set_personality(&(ex))
 
+struct mm_struct;
 extern unsigned long arch_randomize_brk(struct mm_struct *mm);
 #define arch_randomize_brk arch_randomize_brk