cpack: Fix installed size computation with PackageMaker generator

Use the CPACK_PACKAGING_INSTALL_PREFIX when looking for files in a
component.  Otherwise we report all packages having size 1.

Source/CPack/cmCPackPackageMakerGenerator.cxx

@@ -964,6 +964,7 @@ cmCPackPackageMakerGenerator::CreateChoice(const cmCPackComponent& component,
   std::string dirName = this->GetOption("CPACK_TEMPORARY_DIRECTORY");
   dirName += '/';
   dirName += component.Name;
+  dirName += this->GetOption("CPACK_PACKAGING_INSTALL_PREFIX");
   unsigned long installedSize
     = component.GetInstalledSizeInKbytes(dirName.c_str());