Adding Fields to ListOptions in networking

Change-Id: I688e8993a9d8a853ee6bd040f6d704391447f9d5
diff --git a/openstack/networking/v2/ports/requests.go b/openstack/networking/v2/ports/requests.go
index 0cb899f..193901b 100644
--- a/openstack/networking/v2/ports/requests.go
+++ b/openstack/networking/v2/ports/requests.go
@@ -17,19 +17,20 @@
 // by a particular port attribute. SortDir sets the direction, and is either
 // `asc' or `desc'. Marker and Limit are used for pagination.
 type ListOpts struct {
-	Status       string `q:"status"`
-	Name         string `q:"name"`
-	AdminStateUp *bool  `q:"admin_state_up"`
-	NetworkID    string `q:"network_id"`
-	TenantID     string `q:"tenant_id"`
-	DeviceOwner  string `q:"device_owner"`
-	MACAddress   string `q:"mac_address"`
-	ID           string `q:"id"`
-	DeviceID     string `q:"device_id"`
-	Limit        int    `q:"limit"`
-	Marker       string `q:"marker"`
-	SortKey      string `q:"sort_key"`
-	SortDir      string `q:"sort_dir"`
+	Status       string   `q:"status"`
+	Name         string   `q:"name"`
+	AdminStateUp *bool    `q:"admin_state_up"`
+	NetworkID    string   `q:"network_id"`
+	TenantID     string   `q:"tenant_id"`
+	DeviceOwner  string   `q:"device_owner"`
+	MACAddress   string   `q:"mac_address"`
+	ID           string   `q:"id"`
+	DeviceID     string   `q:"device_id"`
+	Limit        int      `q:"limit"`
+	Marker       string   `q:"marker"`
+	SortKey      string   `q:"sort_key"`
+	SortDir      string   `q:"sort_dir"`
+	Fields       []string `q:"fields"`
 }
 
 // ToPortListQuery formats a ListOpts into a query string.