mtd: fix physmap.h warnings

Fix build warnings in physmap.h:

include/linux/mtd/physmap.h:25: warning: 'struct platform_device' declared inside parameter list
include/linux/mtd/physmap.h:25: warning: its scope is only this definition or declaration, which is probably not what you want
include/linux/mtd/physmap.h:26: warning: 'struct platform_device' declared inside parameter list
include/linux/mtd/physmap.h:27: warning: 'struct platform_device' declared inside parameter list

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

include/linux/mtd/physmap.h

@@ -19,6 +19,7 @@
 #include <linux/mtd/partitions.h>
 
 struct map_info;
+struct platform_device;
 
 struct physmap_flash_data {
 	unsigned int		width;