swig bindings: typecast the thunks for the callback macros.

SWIG deliberately drops the qualifiers from the type for the temporary
variable it uses.  This generates warnings.  So let's just cast to
the proper type so we don't get warnings about dropping qualifiers.

* subversion/bindings/swig/include/svn_types.swg
  (%callback_typemap, %callback_typemap_maybenull): Add casts to
    the type of the variable we're assinging to when setting the thunk.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1598085 13f79535-47bb-0310-9956-ffa450edef68