VisualStudio10: Initialize the LongestSource at generate time.

There is no need to initialize it at compute time.

Source/cmGlobalVisualStudio10Generator.h

@@ -45,8 +45,6 @@ public:
     std::vector<std::string> const& makeOptions = std::vector<std::string>()
     );
 
-  virtual bool Compute();
-
   ///! create the correct local generator
   virtual cmLocalGenerator *CreateLocalGenerator(cmMakefile* mf);