Debian build environment is not a git repo

--- a/webauthn.gemspec
+++ b/webauthn.gemspec
@@ -22,10 +22,7 @@
     "source_code_uri" => "https://github.com/cedarcode/webauthn-ruby"
   }
 
-  spec.files =
-    `git ls-files -z`.split("\x0").reject do |f|
-      f.match(%r{^(test|spec|features|assets)/})
-    end
+  spec.files = Dir['lib/**/*']
 
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }