platform/chrome: cros_ec: fix compilation warning

When including cros_ec.h solely, the compiler emits the following
warning:
> 'struct cros_ec_device' declared inside parameter list
  will not be visible outside of this definition or declaration

Fix it by forward declaration.
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20231003080453.4011301-1-tzungbi@kernel.orgSigned-off-by: Tzung-Bi Shih <tzungbi@kernel.org>

drivers/platform/chrome/cros_ec.h

@@ -10,6 +10,8 @@
 
 #include <linux/interrupt.h>
 
+struct cros_ec_device;
+
 int cros_ec_register(struct cros_ec_device *ec_dev);
 void cros_ec_unregister(struct cros_ec_device *ec_dev);