Silence integer conversion warnings in FSX.

* subversion/libsvn_fs_x/reps.c
  (add_new_text,
   svn_fs_x__reps_add, 
   get_text,
   svn_fs_x__read_reps_container): Cast sizes to 32 bits.  We already made
                                   sure that no size exceeds 2GB.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1616604 13f79535-47bb-0310-9956-ffa450edef68