bazel: fix http_archive for bazel 0.21
diff --git a/WORKSPACE b/WORKSPACE
index 74e276d..67e876d 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -22,6 +22,7 @@
 
 # Go rules
 
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 http_archive(
     name = "io_bazel_rules_go",
     url = "https://github.com/bazelbuild/rules_go/releases/download/0.16.5/rules_go-0.16.5.tar.gz",