Fix building SWIG bindings with release mode on OS X (i.e. out of the tarball).

When ./autogen.sh --release is used we generate the SWIG interface then rather
than at build time.  This allows users to use SWIG bindings without needing
SWIG installed.  However, we were trying to hide the gnome keyring callback
helper at SWIG interface generation time that may not be available at build
time.

There is no reason to hide the definitions needed for this helper on platforms
without gnome-keyring support since there is actually nothing tied to the
gnome keyring being available when they are used.  If the gnome keyring auth 
provider isn't available or enabled the callback will simply never be called.

* subversion/include/svn_auth.h
  (svn_auth_gnome_keyring_unlock_prompt_func_t,
   SVN_AUTH_PARAM_GNOME_KEYRING_UNLOCK_PROMPT_FUNC,
   SVN_AUTH_PARAM_GNOME_KEYRING_UNLOCK_PROMPT_BATON): Make available regardless
     of the platform by moving outside of the #if block.

* subversion/bindings/swig/core.i
  (svn_auth_set_gnome_keyring_unlock_prompt_func): Remove the unnecessary
    #ifdef block around this %inline.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1543961 13f79535-47bb-0310-9956-ffa450edef68