Add internal.bzl to EXTRA_DIST. (#8663)

internal.bzl is required to load java/core/BUILD and java/lite/BUILD, so it must be distributed.
diff --git a/Makefile.am b/Makefile.am
index 8cf5473..573003b 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1427,7 +1427,8 @@
   third_party/zlib.BUILD                 \
   third_party/wyhash/LICENSE             \
   third_party/wyhash/wyhash.h            \
-  util/python/BUILD
+  util/python/BUILD                      \
+  internal.bzl
 
 
 # Deletes all the files generated by autogen.sh.