ARC: fpu: fix randconfig build error reported by 0-day test service

Reported-by: kbuild test robot <lkp@intel.com>
Link: http://lists.infradead.org/pipermail/linux-snps-arc/2020-February/006845.htmlSigned-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/fpu.h

@@ -43,6 +43,8 @@ extern void fpu_init_task(struct pt_regs *regs);
 
 #endif	/* !CONFIG_ISA_ARCOMPACT */
 
+struct task_struct;
+
 extern void fpu_save_restore(struct task_struct *p, struct task_struct *n);
 
 #else	/* !CONFIG_ARC_FPU_SAVE_RESTORE */