Allow compiling from source on windows:

- I'm trying to compile Prism from source on a Windows machine but
  it unforunately fails when trying to render the template due to the
  shebang that doesn't exist on Windows.

  I modified the `system` call to call ruby with its full path
  directly instead.

Author: Edouard-chin

ext/prism/extconf.rb

@@ -37,7 +37,7 @@
 def generate_templates
   Dir.chdir(File.expand_path("../..", __dir__)) do
     if !File.exist?("include/prism/ast.h") && Dir.exist?(".git")
-      system("templates/template.rb", exception: true)
+      system(RbConfig.ruby, "templates/template.rb", exception: true)
     end
   end
 end