st/vdpau: Link vdpau targets with librt to avoid unresolved symbols error related...

st/vdpau: Link vdpau targets with librt to avoid unresolved symbols error related to the use of the clock_gettime() function

This patch fix a "Unresolved Symbols" run time error when using G3DVL
through the VDPAU state tracker, by linking the vdpau targets with librt.
Reported by Arkadiusz Miśkiewicz.

Caused by this commit :
commit e911dbb56374edf9f3b7c4cec0cf9a22738bb198
Author: Emeric Grange <emeric.grange@gmail.com>
Date:   Mon Sep 12 23:39:33 2011 +0200

Signed-off-by: Emeric Grange <emeric.grange@gmail.com>