drivers/char: Fix implicit use of err.h in ramoops.c

drivers/char/ramoops.c: In function 'ramoops_init':
drivers/char/ramoops.c:221: error: implicit declaration of function 'IS_ERR'
drivers/char/ramoops.c:222: error: implicit declaration of function 'PTR_ERR'
make[3]: *** [drivers/char/ramoops.o] Error 1

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

drivers/char/ramoops.c

···
 #include <linux/module.h>
 #include <linux/kmsg_dump.h>
 #include <linux/time.h>
+#include <linux/err.h>
 #include <linux/io.h>
 #include <linux/ioport.h>
 #include <linux/platform_device.h>