Use Dir.chdir for the location of gemspec

The current glob pattern caused to issue with Ruby package installation.

```
/Users/hsbt/Documents/github.com/ruby/ruby/lib/fileutils.rb:1625:in 'File.stat': No such file or directory @ rb_file_s_stat - /Users/hsbt/Documents/github.com/ruby/ruby/.bundle/gems/rdoc-6.14.0/lib/English.rb (Errno::ENOENT)
        from /Users/hsbt/Documents/github.com/ruby/ruby/lib/fileutils.rb:1625:in 'block in FileUtils#install'
        from /Users/hsbt/Documents/github.com/ruby/ruby/lib/fileutils.rb:2471:in 'block in FileUtils#fu_each_src_dest'
        from /Users/hsbt/Documents/github.com/ruby/ruby/lib/fileutils.rb:2487:in 'FileUtils#fu_each_src_dest0'
        from /Users/hsbt/Documents/github.com/ruby/ruby/lib/fileutils.rb:2469:in 'FileUtils#fu_each_src_dest'
        from /Users/hsbt/Documents/github.com/ruby/ruby/lib/fileutils.rb:1624:in 'FileUtils#install'
        from ./tool/rbinstall.rb:216:in 'Object#install'
        from ./tool/rbinstall.rb:694:in 'block in RbInstall::DirPackage#extract_files'
        from ./tool/rbinstall.rb:689:in 'Array#each'
        from ./tool/rbinstall.rb:689:in 'RbInstall::DirPackage#extract_files'
        from /Users/hsbt/Documents/github.com/ruby/ruby/lib/rubygems/installer.rb:853:in 'Gem::Installer#extract_files'
        from /Users/hsbt/Documents/github.com/ruby/ruby/lib/rubygems/installer.rb:312:in 'Gem::Installer#install'
        from ./tool/rbinstall.rb:746:in 'block in RbInstall::UnpackedInstaller#install'
        from ./tool/rbinstall.rb:534:in 'RbInstall.no_write'
        from ./tool/rbinstall.rb:746:in 'RbInstall::UnpackedInstaller#install'
        from ./tool/rbinstall.rb:1174:in 'block (2 levels) in <main>'
        from ./tool/rbinstall.rb:1138:in 'IO.foreach'
        from ./tool/rbinstall.rb:1138:in 'block in <main>'
        from ./tool/rbinstall.rb:1224:in 'block in <main>'
        from ./tool/rbinstall.rb:1221:in 'Array#each'
        from ./tool/rbinstall.rb:1221:in '<main>'
```

Author: hsbt

rdoc.gemspec

@@ -54,7 +54,9 @@ RDoc includes the +rdoc+ and +ri+ tools for generating and displaying documentat
     "rdoc.gemspec",
   ]
   template_files = Dir.glob("lib/rdoc/generator/template/**/*")
-  lib_files = Dir.glob("lib/**/*.{rb,kpeg,ry}")
+  lib_files = Dir.chdir(File.expand_path('..', __FILE__)) do
+    Dir.glob("lib/**/*.{rb,kpeg,ry}")
+  end
 
   s.files = (non_lib_files + template_files + lib_files).uniq