test_arc4random: Fix warning.

The result of a function being tested is discarded, which produces a
"set but not used" warning from some compilers.  We add a void cast to
avoid this.

TESTED:
Previously present warnings are now gone.

test/test_arc4random.c

@@ -64,7 +64,7 @@ bench(int fd, size_t siz, size_t niter)
         s0 = sys_cpu_timestamp();
         arc4random_buf(buf, siz);
         s1 = sys_cpu_timestamp();
-        m  =  read(fd, buf, siz);
+        m  =  read(fd, buf, siz); (void) m;
         s2 = sys_cpu_timestamp();
 
         ta += s1 - s0;