Fix linter warning.

Author: aardvark179

lib/truffle/truffle/cext.rb

@@ -734,7 +734,7 @@ def rb_enc_to_index(enc)
   end
 
   def rb_define_dummy_encoding(name)
-    enc, idx = Truffle::EncodingOperations.dummy_encoding(name)
+    _, idx = Truffle::EncodingOperations.dummy_encoding(name)
     idx
   end