DocBook: Reduce noise from make cleandocs

I've got a harmless warning when running make cleandocs on an already
cleaned tree:
  Documentation/DocBook/media/Makefile:28: recipe for target 'cleanmediadocs' failed
  make[1]: [cleanmediadocs] Error 1 (ignored)

Suppress this by passing -f to rm.
Acked-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/DocBook/media/Makefile

@@ -25,7 +25,7 @@ GENFILES := $(addprefix $(MEDIA_OBJ_DIR)/, $(MEDIA_TEMP))
 PHONY += cleanmediadocs
 
 cleanmediadocs:
-	-@rm `find $(MEDIA_OBJ_DIR) -type l` $(GENFILES) $(OBJIMGFILES) 2>/dev/null
+	-@rm -f `find $(MEDIA_OBJ_DIR) -type l` $(GENFILES) $(OBJIMGFILES) 2>/dev/null
 
 $(obj)/media_api.xml: $(GENFILES) FORCE