make clean: also remove generated image files

Change-Id: I80798e79b4ccee64f26f58f9754de02b2958e33e
diff --git a/doc/manuals/Makefile b/doc/manuals/Makefile
index ef80327..6b4ecb8 100644
--- a/doc/manuals/Makefile
+++ b/doc/manuals/Makefile
@@ -25,6 +25,7 @@
 clean:
 	rm -rf $(cleanfiles)
 	rm -rf gen-vty-docbook
+	rm -rf osmopcu-usermanual__*.{svg,png}
 
 gen-vty-docbook: FORCE
 	$(call command,xsltproc -o generated/combined1.xml \