ci: fix test-gem-file-contents source file test

Author: flavorjones

bin/test-gem-file-contents

@@ -116,7 +116,7 @@ describe File.basename(gemfile) do
       end
 
       it "contains C source files in src/" do
-        assert_operator(gemfile_contents.count { |f| File.fnmatch?("src/**/*.c", f) }, :>=, 10)
+        assert_operator(gemfile_contents.count { |f| File.fnmatch?("src/*.c", f) }, :>=, 10)
       end
 
       it "contains extconf.rb" do