libubi: remove private kernel header from includes

libubi.h includes ubi-media.h which was made private in the kernel a
long time ago. There are users of libubi.h, e.g. swupdate, which have to
have ubi-media.h available at build time with this inclusion.

However, libubi.h uses only one symbol from ubi-media.h. Define that symbol
in the header to enable using libubi.h without installing ubi-media.h.

Make up for the transitive symbol use in ubiformat.c by including ubi-media.h.

Signed-off-by: Bastian Germann <bastiangermann@fishpost.de>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>