Exclude extconf.rb from rdoc

Author: shugo

Rakefile

@@ -7,7 +7,7 @@ begin
   RDoc::Task.new do |doc|
     doc.main   = "README.md"
     doc.title  = "Curses: A Ruby binding for curses, ncurses, and PDCurses."
-    doc.rdoc_files = FileList.new %w[ext lib README.md BSDL COPYING]
+    doc.rdoc_files = FileList.new %w[ext/curses/curses.c lib README.md BSDL COPYING]
     doc.rdoc_dir = "_site" # for github pages
   end
 rescue LoadError