drm-howto: add _FILE_OFFSET_BITS=64

We need off_t to be 64bit, so define _FILE_OFFSET_BITS. Otherwise, the
examples fail on 32bit systems with non-intel drivers that use the lower
address-space for TTM internal memory. This is often done to get a linear
mapping with all available memory, not only a given resource.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>