vendor: Update github.com/gogo/protobuf, keeping all files

gvt fetch -a, because we need the protobuf files etc for regeneration

vendor/github.com/gogo/protobuf/test/issue260/models.go

@@ -0,0 +1,19 @@
+package issue260
+
+import "time"
+
+type Dropped struct {
+	Name string
+	Age  int32
+}
+
+func (d *Dropped) Drop() bool {
+	return true
+}
+
+type DroppedWithoutGetters struct {
+	Width             int64
+	Height            int64
+	Timestamp         time.Time  `protobuf:"bytes,3,opt,name=timestamp,stdtime" json:"timestamp"`
+	NullableTimestamp *time.Time `protobuf:"bytes,4,opt,name=nullable_timestamp,json=nullableTimestamp,stdtime" json:"nullable_timestamp,omitempty"`
+}