Documentation: add cleanmediadocs to the documentation targets

This was broken when updating the documentation targets for the Sphinx
build, and moving from %docs target pattern to explicitly listed
targets.

Cc: Markus Heiser <markus.heiser@darmarit.de>
Cc: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Fixes: 22cba31bae9d ("Documentation/sphinx: add basic working Sphinx configuration and build")
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Makefile b/Makefile
index f7f393c..34bc4e2 100644
--- a/Makefile
+++ b/Makefile
@@ -1414,7 +1414,7 @@
 
 # Documentation targets
 # ---------------------------------------------------------------------------
-DOC_TARGETS := xmldocs sgmldocs psdocs pdfdocs htmldocs mandocs installmandocs epubdocs cleandocs
+DOC_TARGETS := xmldocs sgmldocs psdocs pdfdocs htmldocs mandocs installmandocs epubdocs cleandocs cleanmediadocs
 PHONY += $(DOC_TARGETS)
 $(DOC_TARGETS): scripts_basic FORCE
 	$(Q)$(MAKE) $(build)=scripts build_docproc build_check-lc_ctype