Add go directive to indicate go version number.

Test: go vet ./...
Change-Id: I815bec9cba04f6c171f0ef5a3a909c4f5f28f6c2
diff --git a/go.mod b/go.mod
index cc328e0..1483a31 100644
--- a/go.mod
+++ b/go.mod
@@ -7,3 +7,5 @@
 replace github.com/golang/protobuf v0.0.0 => ../../external/golang-protobuf
 
 replace github.com/google/blueprint v0.0.0 => ../blueprint
+
+go 1.13