fix: reference gax import in storage libs (#2720)

The storage client is the only client that imports gax. Depending
on the RPCs being generated it may or may not use the gax import.
To avoid generating an unreferenced lets always import it.

Author: codyoss

google-api-go-generator/gen.go

@@ -858,6 +858,9 @@ func (a *API) GenerateCode() ([]byte, error) {
 	pn("var _ = context.Canceled")
 	pn("var _ = internaloption.WithDefaultEndpoint")
 	pn("var _ = internal.Version")
+	if a.Name == "storage" {
+		pn("var _ = gax.Version")
+	}
 	pn("")
 	pn("const apiId = %q", a.doc.ID)
 	pn("const apiName = %q", a.doc.Name)