mtdchar build fix

sh:

drivers/mtd/mtdchar.c: In function `mtd_mmap':
drivers/mtd/mtdchar.c:817: error: dereferencing pointer to incomplete type
drivers/mtd/mtdchar.c:817: error: `VM_SHARED' undeclared (first use in this function)
drivers/mtd/mtdchar.c:817: error: (Each undeclared identifier is reported only once

Cc: David Woodhouse <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/mtd/mtdchar.c

···
 
 #include <linux/device.h>
 #include <linux/fs.h>
+#include <linux/mm.h>
 #include <linux/err.h>
 #include <linux/init.h>
 #include <linux/kernel.h>