arm: fix a warning

qemu/target-arm/helper.c

@@ -5661,6 +5661,8 @@ static bool round_to_inf(float_status *fpst, bool sign_bit)
         return sign_bit;
     case float_round_to_zero: /* Round to Zero */
         return false;
+    default:
+        break;
     }
 
     g_assert_not_reached();