Documentation: Fix DocBook build with relative $(srctree)

After commits 890676c6 (kbuild: Use relative path when building in the source
tree) and 9da0763b

 (kbuild: Use relative path when building in a subdir
of the source tree), the $(srctree) variable can be a relative path.
This breaks Documentation/DocBook/media/Makefile, because it tries to
create symlinks from a subdirectory of the object tree to the source
tree. Fix this by using a full path in this case.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>