Add missing include blockers and 'extern "C"' statements to FSX headers.

* subversion/libsvn_fs_x/low_level.h
  subversion/libsvn_fs_x/revprops.h
  (): add header controls

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1509952 13f79535-47bb-0310-9956-ffa450edef68