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>

Documentation/DocBook/media/Makefile

@@ -202,8 +202,8 @@ $(MEDIA_OBJ_DIR)/%: $(MEDIA_SRC_DIR)/%.b64
 
 $(MEDIA_OBJ_DIR)/v4l2.xml: $(OBJIMGFILES)
 	@$($(quiet)gen_xml)
-	@(ln -sf $(MEDIA_SRC_DIR)/v4l/*xml $(MEDIA_OBJ_DIR)/)
-	@(ln -sf $(MEDIA_SRC_DIR)/dvb/*xml $(MEDIA_OBJ_DIR)/)
+	@(ln -sf `cd $(MEDIA_SRC_DIR) && /bin/pwd`/v4l/*xml $(MEDIA_OBJ_DIR)/)
+	@(ln -sf `cd $(MEDIA_SRC_DIR) && /bin/pwd`/dvb/*xml $(MEDIA_OBJ_DIR)/)
 
 $(MEDIA_OBJ_DIR)/videodev2.h.xml: $(srctree)/include/uapi/linux/videodev2.h $(MEDIA_OBJ_DIR)/v4l2.xml
 	@$($(quiet)gen_xml)