Add missing dependency

Author: abeverley

Makefile.PL

@@ -32,6 +32,7 @@ WriteMakefile(
         'LWP::Protocol::https'       => 0,
         'LWP::UserAgent'             => 0,
         'Moo'                        => 0,
+        'MooX::Singleton'            => 0,
         'MooX::Types::MooseLike'     => 0,
         'Term::ReadKey'              => 0,
         'Test::More'                 => 0,