drbd: check MODULE for THIS_MODULE

THIS_MODULE is NULL only when drbd is compiled as built-in,
so the #ifdef CONFIG_MODULES should be #ifdef MODULE instead.

This fixes the warning:

drivers/block/drbd/drbd_main.c: In function ‘drbd_buildtag’:
drivers/block/drbd/drbd_main.c:4187:24: warning: the comparison will always evaluate as ‘true’ for the address of ‘__this_module’ will never be NULL [-Waddress]

Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>