thermal: sysfs: remove symbols of emul_temp when config is disabled

Remove the following warning:
In file included from drivers/thermal/thermal_sysfs.c:19:0:
include/linux/device.h:575:26: warning: ‘dev_attr_emul_temp’ defined but not used [-Wunused-variable]
  struct device_attribute dev_attr_##_name = __ATTR(_name, _mode, _show, _store)
                          ^
drivers/thermal/thermal_sysfs.c:395:8: note: in expansion of macro ‘DEVICE_ATTR’

when emul temp is disabled at Kconfig.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>