Hexagon: fix signal.c compile error

Signed-off-by: Richard Kuo <rkuo@codeaurora.org>

arch/hexagon/include/uapi/asm/signal.h

@@ -19,6 +19,8 @@
 #ifndef _ASM_SIGNAL_H
 #define _ASM_SIGNAL_H
 
+#include <uapi/asm/registers.h>
+
 extern unsigned long __rt_sigtramp_template[2];
 
 void do_signal(struct pt_regs *regs);