Add a simple proto_cc_library rule

It only supports a single .proto file without any dependencies, but
that's enough for now.

Change-Id: Ibd80c8594b90956c3c5b0b46db10cf991d7ed1ad
diff --git a/third_party/protobuf/BUILD b/third_party/protobuf/BUILD
index bb5d889..6686800 100644
--- a/third_party/protobuf/BUILD
+++ b/third_party/protobuf/BUILD
@@ -171,6 +171,12 @@
 
 WELL_KNOWN_PROTOS = ["src/" + s for s in RELATIVE_WELL_KNOWN_PROTOS]
 
+filegroup(
+    name = "well_known_protos",
+    srcs = WELL_KNOWN_PROTOS,
+    visibility = ["//visibility:public"],
+)
+
 cc_proto_library(
     name = "cc_wkt_protos",
     srcs = WELL_KNOWN_PROTOS,