thermal: Fix build warning of !defined(CONFIG_THERMAL_OF)

Add "inline" to thermal_zone_of_get_sensor_id() function to avoid below
build warning of !defined(CONFIG_THERMAL_OF).

In file included from drivers/hwmon/hwmon.c:22:
include/linux/thermal.h:382:12: warning: 'thermal_zone_of_get_sensor_id'
defined but not used [-Wunused-function]
382 | static int thermal_zone_of_get_sensor_id(struct device_node *tz_np,

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1583222684-10229-1-git-send-email-Anson.Huang@nxp.com