Add load() for proto_library to @proto_cross_repo_boundary

Missed that one in bazel-contrib#989.

Author: Yannic

test/proto_cross_repo_boundary/repo/BUILD.repo

@@ -1,5 +1,7 @@
+load("@rules_proto//proto:defs.bzl", "proto_library")
+
 proto_library(
     name = "sample_proto",
     srcs = ["sample.proto"],
     visibility = ["//visibility:public"],
-)
+)