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 <paul.gortmaker@windriver.com>