drivers/char: Add export.h for EXPORT_SYMBOL/THIS_MODULE as required

They will need it called out explicitly in the near future due
to a module.h usage cleanup that removes its implicit presence
everywhere.

Signed-off-by: Paul Gortmaker <[e-mail]>

drivers/char/mem.c

···
 #include <linux/bootmem.h>
 #include <linux/splice.h>
 #include <linux/pfn.h>
+#include <linux/export.h>
 
 #include <asm/uaccess.h>
 #include <asm/io.h>

drivers/char/ttyprintk.c

···
 #include <linux/device.h>
 #include <linux/serial.h>
 #include <linux/tty.h>
+#include <linux/export.h>
 
 struct ttyprintk_port {
 	struct tty_port port;