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

Commit 5240f4e68d42 ("[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.

---
This error happens for me even on a 'make cleandocs; make htmldocs', so I'm not sure how it got through - maybe if you only rebuild a single file each time then it does not show up. Odd, maybe it is a specific thing on my system, but I can't think of anything that would cause that?

 Documentation/DocBook/media/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 08527e7ea4d0..02848146fc3a 100644
@@ -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

2.4.3