Make the prototypes for svn_dbg__ functions visible when
compiling the definitions.

* subversion/include/private/svn_debug.h: Wrap prototypes in
  in SVN_DBG__PROTOTYPES, rather than SVN_DEBUG, and define it
  when SVN_DEBUG is defined.

* subversion/libsvn_subr/debug.c: Ensure SVN_DBG__PROTOTYPES is defined.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1459421 13f79535-47bb-0310-9956-ffa450edef68