win32/mkexports.rb: do not export internal symbols

Functions using `rb_thread_t` and `rb_execution_context_t` are
internal use only.

win32/mkexports.rb

@@ -7,7 +7,7 @@ module RbConfig
 end
 
 class Exports
-  PrivateNames = /(?:Init_|InitVM_|ruby_static_id_|DllMain\b)/
+  PrivateNames = /(?:Init_|InitVM_|ruby_static_id_|threadptr|_ec_|DllMain\b)/
 
   @@subclass = []
   def self.inherited(klass)