Avoid a compilation warning in eval.c

* src/eval.c (specpdl_unrewind): Avoid a compilation warning.

src/eval.c

@@ -4106,7 +4106,7 @@ specpdl_unrewind (union specbinding *pdl, int distance, bool vars_only)
                  non-local, this is fine, but if it ever reverts to being
                  local we may end up using this entry "in the wrong
                  direction".  */
-	      ;
+	      {};
 	  }
 	  break;
 	}