dix: rpcbuf: export x_rpcbuf_write_binary_pad() for GLX

Mark x_rpcbuf_write_binary_pad() as _X_EXPORT, like the other rpcbuf
helpers already used by the GLX module (which is built as a separate
shared object). Needed by upcoming GLX reply conversions that emit
string / raw binary payloads through the rpcbuf reply path. Not part of
the public ABI.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

dix/rpcbuf.c

@@ -236,6 +236,7 @@ Bool x_rpcbuf_write_CARD64s(x_rpcbuf_t *rpcbuf, const CARD64 *values,
     return TRUE;
 }
 
+_X_EXPORT /* only for GLX, not part of public ABI */
 Bool x_rpcbuf_write_binary_pad(x_rpcbuf_t *rpcbuf, const void *values,
                                size_t size)
 {