auxdisplay: charlcd: add include guard to charlcd.h

Add a header include guard just in case.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>

drivers/auxdisplay/charlcd.h

@@ -6,6 +6,9 @@
  * Copyright (C) 2016-2017 Glider bvba
  */
 
+#ifndef _CHARLCD_H
+#define _CHARLCD_H
+
 struct charlcd {
 	const struct charlcd_ops *ops;
 	const unsigned char *char_conv;	/* Optional */
@@ -37,3 +40,5 @@ int charlcd_register(struct charlcd *lcd);
 int charlcd_unregister(struct charlcd *lcd);
 
 void charlcd_poke(struct charlcd *lcd);
+
+#endif /* CHARLCD_H */