cmTarget: Make GetInterfaceLinkLibraries const.

Source/cmTarget.cxx

@@ -811,7 +811,7 @@ void cmTarget::GetDirectLinkLibraries(const char *config,
 
 //----------------------------------------------------------------------------
 void cmTarget::GetInterfaceLinkLibraries(const char *config,
-                            std::vector<std::string> &libs, cmTarget *head)
+                        std::vector<std::string> &libs, cmTarget *head) const
 {
   const char *prop = this->GetProperty("INTERFACE_LINK_LIBRARIES");
   if (prop)

Source/cmTarget.h

@@ -186,7 +186,7 @@ public:
                               cmTarget const* head) const;
   void GetInterfaceLinkLibraries(const char *config,
                               std::vector<std::string> &,
-                              cmTarget *head);
+                              cmTarget *head) const;
 
   /** Compute the link type to use for the given configuration.  */
   LinkLibraryType ComputeLinkType(const char* config) const;