[ruby/stringio] Do not compile the C extension on TruffleRuby

* Before this it was compiled but not used, because TruffleRuby has
  a stringio.rb in stdlib and .rb has precedence over .so.
  In fact that extension never worked on TruffleRuby,
  because rb_io_extract_modeenc() has never been defined on TruffleRuby.
* So this just skip compiling the extension since compilation of it now fails:
  ruby/openssl#699

ruby/stringio@d791b63df6

ext/stringio/extconf.rb

@@ -1,3 +1,7 @@
 # frozen_string_literal: false
 require 'mkmf'
-create_makefile('stringio')
+if RUBY_ENGINE == 'ruby'
+  create_makefile('stringio')
+else
+  File.write('Makefile', dummy_makefile("").join)
+end