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: NRalf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/elf.h

@@ -310,6 +310,7 @@ do {									\
 
 #endif /* CONFIG_64BIT */
 
+struct pt_regs;
 struct task_struct;
 
 extern void elf_dump_regs(elf_greg_t *, struct pt_regs *regs);