Fix singleton method definition style

Author: sambostock

lib/tapioca/dsl/compilers/rubocop.rb

@@ -14,9 +14,12 @@ module Compilers
       class RuboCop < Compiler
         ConstantType = type_member { { fixed: T.all(T.class_of(::RuboCop::Cop::Base), Extensions::RuboCop) } }
 
-        sig { override.returns(T::Enumerable[Class]) }
-        def self.gather_constants
-          descendants_of(::RuboCop::Cop::Base).select { |constant| name_of(constant) }
+        class << self
+          extend T::Sig
+          sig { override.returns(T::Enumerable[Class]) }
+          def gather_constants
+            descendants_of(::RuboCop::Cop::Base).select { |constant| name_of(constant) }
+          end
         end
 
         sig { override.void }