Add proper header file for fewer warnings in blkmtd.c

It's still broken, but now the silly warnings no longer hide
the _real_ problems in this file.

drivers/mtd/devices/blkmtd.c

@@ -45,6 +45,7 @@
 
 #include <linux/config.h>
 #include <linux/module.h>
+#include <linux/init.h>
 
 #include <linux/fs.h>
 #include <linux/blkdev.h>