Another fix for google import.

BUILD

diff --git a/BUILD b/BUILD
index b3a163f..c85ece9 100644
--- a/BUILD
+++ b/BUILD
@@ -124,6 +124,7 @@
     name = "reflection",
     srcs = [
         "upb/def.c",
+        "upb/msg.h",
         "upb/reflection.c",
     ],
     hdrs = [