MIPS: Fix elfcore.c build warning

kernel/elfcore.c includes <linux/elf.h> which includes the <asm/elf.h>.  In
<asm/elf.h>, struct pt_regs is declared inside the parameter list of the
elf_dump_regs function which causes a kernel build warning.

Fixed by adding a forward declaration of struct pt_regs.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>