fix 'make clean': shell glob, ignore failure

Unfortunately a glob like osmo-x__*.{svg,png} doesn't work, so have the
suffixes in separate globs.

Add dashes to indicate that failure should be ignored.

Change-Id: I6bc4d9ea72b43a573acbc860c23397f748de2c7b
diff --git a/doc/manuals/Makefile b/doc/manuals/Makefile
index 7df5168..6f7d28d 100644
--- a/doc/manuals/Makefile
+++ b/doc/manuals/Makefile
@@ -22,8 +22,9 @@
 osmosgsn-usermanual.pdf: chapters/*.adoc
 
 clean:
-	rm -rf osmosgsn-usermanual__*.{svg,png,check}
-	rm -rf $(cleanfiles)
+	-rm -rf $(cleanfiles)
+	-rm osmosgsn-usermanual__*.svg osmosgsn-usermanual__*.png
+	-rm osmosgsn-usermanual.check
 
 gen-sgsn-vty-docbook: FORCE
 	$(call command,xsltproc -o generated/combined1.xml \