test: add base64 as a dependency

This is required from ruby 3.4 onwards.

Author: carlosmn

rugged.gemspec

@@ -31,5 +31,6 @@ desc
   s.add_development_dependency "rake-compiler", ">= 0.9.0"
   s.add_development_dependency "pry"
   s.add_development_dependency "minitest", "~> 5.0"
+  s.add_development_dependency "base64"
   s.metadata["msys2_mingw_dependencies"] = "libssh2 cmake"
 end