import ietf yang types into portchannel

Author: bradh352

src/sonic-yang-models/yang-models/sonic-portchannel.yang

@@ -9,6 +9,10 @@ module sonic-portchannel {
         prefix stypes;
     }
 
+    import ietf-yang-types {
+        prefix yang;
+    }
+
     import sonic-extension {
         prefix ext;
     }