libvirt_private.syms: add virProcessGetStartTime

https://bugzilla.redhat.com/show_bug.cgi?id=973543
(cherry picked from commit cbb3b71d)

src/libvirt_private.syms

@@ -1711,6 +1711,7 @@ virPortAllocatorRelease;
 virProcessAbort;
 virProcessGetAffinity;
 virProcessGetNamespaces;
+virProcessGetStartTime;
 virProcessKill;
 virProcessKillPainfully;
 virProcessSetAffinity;