Add support for leafrefs

Change-Id: I2286aaa96abd960d13bb0b04a778fb264b2f74a4
diff --git a/src/utils.cpp b/src/utils.cpp
index d06d35b..979bda4 100644
--- a/src/utils.cpp
+++ b/src/utils.cpp
@@ -62,6 +62,8 @@
         return "an enum";
     case yang::LeafDataTypes::IdentityRef:
         return "an identity";
+    case yang::LeafDataTypes::LeafRef:
+        return "a leafref";
     default:
         throw std::runtime_error("leafDataTypeToString: unsupported leaf data type");
     }