auxdisplay: charlcd: Provide a forward declaration

While there is no compilation error, strictly speaking the compiler
should know about used types beforehand. Provide a forward declaration
for struct charlcd_ops before using it in struct charlcd.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/auxdisplay/charlcd.h

@@ -36,6 +36,8 @@ enum charlcd_lines {
 	CHARLCD_LINES_2,
 };
 
+struct charlcd_ops;
+
 struct charlcd {
 	const struct charlcd_ops *ops;
 	const unsigned char *char_conv;	/* Optional */