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/build_tests/proto.proto b/build_tests/proto.proto
new file mode 100644
index 0000000..367a650
--- /dev/null
+++ b/build_tests/proto.proto
@@ -0,0 +1,12 @@
+syntax = "proto3";
+
+package frc971;
+
+import "google/protobuf/empty.proto";
+
+message TestProto {
+  string s = 1;
+  int32 i = 2;
+  // Making sure that well-known protos work.
+  .google.protobuf.Empty empty = 3;
+}