cmTarget: Fix CMP0022 OLD breakage from recent refactoring

In commit 7b0834e9 (cmTarget: Refactor internal LinkImplementation map,
2014-06-19) cmTarget::GetLinkImplementationLibrariesInternal was changed
accidentally to pass "this" to ComputeLinkImplementation instead of
"head".  Change it back.

Source/cmTarget.cxx

@@ -6568,7 +6568,7 @@ cmTarget::GetLinkImplementationLibrariesInternal(const std::string& config,
   if(!impl.LibrariesDone)
     {
     impl.LibrariesDone = true;
-    this->ComputeLinkImplementation(config, impl, this);
+    this->ComputeLinkImplementation(config, impl, head);
     }
   return &impl;
 }