propagation: add 1.8 build flags

Change-Id: Ia82fbad6e3bb00bd373c47d88503d25870d1ed0f
Reviewed-on: https://code-review.googlesource.com/33610
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/transport/http/internal/propagation/http.go b/transport/http/internal/propagation/http.go
index f75f9fa..24b4f0d 100644
--- a/transport/http/internal/propagation/http.go
+++ b/transport/http/internal/propagation/http.go
@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// +build go1.8
+
 // Package propagation implements X-Cloud-Trace-Context header propagation used
 // by Google Cloud products.
 package propagation
diff --git a/transport/http/internal/propagation/http_test.go b/transport/http/internal/propagation/http_test.go
index 7bf4d20..f47ef25 100644
--- a/transport/http/internal/propagation/http_test.go
+++ b/transport/http/internal/propagation/http_test.go
@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// +build go1.8
+
 package propagation
 
 import (