mtd: spi-nor: add forward declaration for mtd_info

This header can't actually stand alone, as it relies on the declaration
(but not definition) of struct mtd_info. Let's fix that.
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Tested-by: Joachim Eastwood <manabian@gmail.com>

include/linux/mtd/spi-nor.h

@@ -127,6 +127,8 @@ enum spi_nor_option_flags {
 	SNOR_F_USE_FSR		= BIT(0),
 };
 
+struct mtd_info;
+
 /**
  * struct spi_nor - Structure for defining a the SPI NOR layer
  * @mtd:		point to a mtd_info structure