benchtests: Fix warning in bench-strchr.c

Ensure benchtests compile with trunk GCC.

NEWS

@@ -13,7 +13,8 @@ Major new features:
 
 Deprecated and removed features, and other changes affecting compatibility:
 
-  [Add deprecations, removals and changes affecting compatibility here]
+* Support for dumped heaps has been removed - malloc_set_state() now always
+  returns the error code -1.
 
 Changes to build and runtime requirements:
 

benchtests/bench-strchr.c

@@ -92,6 +92,9 @@ IMPL (generic_strchrnul, 0)
    branch coming we want to test the case where a potential branch in
    strchr can be used to skip a later mispredict because of the
    relationship between the two branches. */
+
+static volatile int res;
+
 static void __attribute_optimization_barrier__
 do_one_rand_plus_branch_test (json_ctx_t *json_ctx, impl_t *impl,
                               const CHAR *s, const CHAR *c)
@@ -111,6 +114,7 @@ do_one_rand_plus_branch_test (json_ctx_t *json_ctx, impl_t *impl,
         }
     }
   TIMING_NOW (stop);
+  res = must_execute;
 
   TIMING_DIFF (cur, start, stop);