Fix Windows build: Ensure that libsvn_subr exports the svn_packed__* API.

* build.conf
  (libsvn_subr): add svn_packed_data.h to msvc-exports

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1509927 13f79535-47bb-0310-9956-ffa450edef68