After doing some performance analysis, it was found that the GTK+ mediaLib code
triggers unnecessary lazy loading of dependent libraries.  The current code
uses RTLD_DEFAULT, RTLD_PROBE, RTLD_NEXT, and RTLD_SELF.  However, RTLD_PROBE
is all that is necessary, and avoids triggering the lazy loading.  So this
commit fixes the code to just use RTLD_PROBE.  (Bug 580678)