Add a BUILD.bazel file for nanopb.

--
MOS_MIGRATED_REVID=118917025
diff --git a/third_party/nanopb/BUILD b/third_party/nanopb/BUILD
new file mode 100644
index 0000000..49f8257
--- /dev/null
+++ b/third_party/nanopb/BUILD
@@ -0,0 +1,9 @@
+licenses(["unencumbered"])
+
+cc_library(
+    name = "nanopb",
+    srcs = glob(["*.c"]),
+    hdrs = glob(["*.h"]),
+    includes = ["."],
+    visibility = ["//visibility:public"],
+)