Fix output formatting.

Author: reitermarkus

bindgen/codegen/macro_def.rs

@@ -210,7 +210,7 @@ impl CodeGenerator for MacroDef {
                             });
                         } else {
                             warn!(
-                                "Unhandled variable-like macro: {} = {:?}",
+                                "Unhandled variable-like macro `{}`: {:?}",
                                 var_macro.name(),
                                 expr
                             );