MIPS: Forward declare struct task_struct to avoid potencial warning.

arch/mips/include/asm/smp-ops.h

@@ -15,6 +15,8 @@
 
 #include <linux/cpumask.h>
 
+struct task_struct;
+
 struct plat_smp_ops {
 	void (*send_ipi_single)(int cpu, unsigned int action);
 	void (*send_ipi_mask)(cpumask_t mask, unsigned int action);