osmo-release.sh: Add quotes to string to fix shellcheck warning

Change-Id: I40376c99d44552d9b8023fca8d319a9312b0fbb0
diff --git a/osmo-release.sh b/osmo-release.sh
index 86b41d8..1e5a203 100755
--- a/osmo-release.sh
+++ b/osmo-release.sh
@@ -4,7 +4,7 @@
 
 if [ "z$REL" = "z" ]; then
 	echo "No REL value specified, defaulting to 'patch' release"
-	REL=patch
+	REL="patch"
 fi
 
 BUMPVER=`command -v bumpversion`