Install gemspec even though no .rb and no .so

When building with --with-static-linked-ext, some exts without rb file
doesn't produce neither .so or .rb under .ext/common. Therefore, change
rbinstall.rb to install gemspec even if there is no .so or .rb for that
case.

tool/rbinstall.rb

@@ -966,7 +966,6 @@ def install_default_gem(dir, srcdir, bindir)
     spec = load_gemspec(src)
     file_collector = RbInstall::Specs::FileCollector.new(src)
     files = file_collector.collect
-    next if files.empty?
     spec.files = files
     spec
   }