Build fix for !CONFIG_GDBSTUB case

Once CONFIG_GDBSTUB not configured, compile will generate error.
Signed-off-by: NXiantao Zhang <xiantao.zhang@intel.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

vl.c

@@ -4350,11 +4350,13 @@ static void tcg_cpu_exec(void)
         }
         if (cpu_can_run(env))
             ret = qemu_cpu_exec(env);
+#ifndef CONFIG_GDBSTUB
         if (ret == EXCP_DEBUG) {
             gdb_set_stop_cpu(env);
             debug_requested = 1;
             break;
         }
+#endif
     }
 }