sh: Tidy compiler warnings for SH-2A build.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
diff --git a/arch/sh/kernel/signal.c b/arch/sh/kernel/signal.c
index 4fc5b40..2d9131f 100644
--- a/arch/sh/kernel/signal.c
+++ b/arch/sh/kernel/signal.c
@@ -23,6 +23,7 @@
 #include <linux/personality.h>
 #include <linux/binfmts.h>
 #include <linux/freezer.h>
+#include <linux/io.h>
 #include <asm/system.h>
 #include <asm/ucontext.h>
 #include <asm/uaccess.h>