Temporary work around to fix compiler warning for set but not used
'res' variable.

e.g.
BUILDSTDERR: bench-malloc-simple.c: In function 'bench':
BUILDSTDERR: bench-malloc-simple.c:89:17: error: variable 'res' set but not used [-Werror=unused-but-set-variable]
BUILDSTDERR:    89 |   unsigned long res;
BUILDSTDERR:       |                 ^~~
BUILDSTDERR: cc1: all warnings being treated as errors

Affects aarch64, armv7hl, and s390x.

diff --git a/benchtests/bench-malloc-simple.c b/benchtests/bench-malloc-simple.c
index 83203ff3187654a1..e449dd5a2e351278 100644
--- a/benchtests/bench-malloc-simple.c
+++ b/benchtests/bench-malloc-simple.c
@@ -90,6 +90,8 @@ bench (unsigned long size)
 
   TIMING_INIT (res);
 
+  (void) res;
+
   for (int t = 0; t <= 3; t++)
     for (int i = 0; i < NUM_ALLOCS; i++)
       {