[MIPS] Delete unneeded pt_regs forward declaration.

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

arch/mips/oprofile/op_impl.h

@@ -10,8 +10,6 @@
 #ifndef OP_IMPL_H
 #define OP_IMPL_H 1
 
-struct pt_regs;
-
 extern int null_perf_irq(void);
 extern int (*perf_irq)(void);