Fix files for gemspec files direct-under lib

Collected `files` lacked `lib` prefix.

1 files changed, 1 insertions, 1 deletions

diff --git a/tool/rbinstall.rb b/tool/rbinstall.rb
index ab3193f9e0..ba0d911fc2 100755
--- a/tool/rbinstall.rb
+++ b/tool/rbinstall.rb
@@ -773,7 +773,7 @@ module RbInstall
                     remove_prefix(prefix, ruby_source)
                   end
                 else
-                  [File.basename(@gemspec, '.gemspec') + '.rb']
+                  [@gemspec[%r[(?:[^/]+/)?[^/]+(?=\.gemspec\z)]] + '.rb']
                 end
 
         case File.basename(@gemspec, ".gemspec")