install-progress: Call the real ::fork() in our fork() method

We basically called ourselves before, creating an endless loop.

Reported-By: clang

apt-pkg/install-progress.h

@@ -38,7 +38,7 @@ namespace Progress {
     */
     virtual void StartDpkg() {};
 
-    virtual pid_t fork() {return fork(); };
+    virtual pid_t fork() {return ::fork(); };
 
     virtual void Pulse() {};
     virtual long GetPulseInterval() {