[media] DocBook/media/Makefile: Do not fail mkdir if dir already exists

Commit 5240f4e6 ("[media] DocBook/media/Makefile: Avoid make htmldocs
to fail") introduced a mkdir which is always called through
install_media_images from the Documentation/DocBook/Makefile htmldocs rule.
If you run 'make htmldocs' more than once you get:

 mkdir: cannot create directory ‘./Documentation/DocBook//media_api’:
  File exists

Add -p to the mkdir to continue no matter if the dir already exists.
Signed-off-by: Graham Whaley <graham.whaley@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

Documentation/DocBook/media/Makefile

@@ -199,7 +199,7 @@ DVB_DOCUMENTED = \
 #
 
 install_media_images = \
-	$(Q)-mkdir $(MEDIA_OBJ_DIR)/media_api; \
+	$(Q)-mkdir -p $(MEDIA_OBJ_DIR)/media_api; \
 	cp $(OBJIMGFILES) $(MEDIA_SRC_DIR)/*.svg $(MEDIA_SRC_DIR)/v4l/*.svg $(MEDIA_OBJ_DIR)/media_api
 
 $(MEDIA_OBJ_DIR)/%: $(MEDIA_SRC_DIR)/%.b64