MAINTAINERS: Add missing userspace thermal tools to the thermal section

Patches related to the tmon which is in tools/thermal are floating around
since years because it is unclear who takes care of it.

Add the missing userspace tools directory related to thermal to fix the
situation.
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

MAINTAINERS

@@ -18358,6 +18358,7 @@ F:	drivers/thermal/
 F:	include/linux/cpu_cooling.h
 F:	include/linux/thermal.h
 F:	include/uapi/linux/thermal.h
+F:	tools/thermal/
 
 THERMAL DRIVER FOR AMLOGIC SOCS
 M:	Guillaume La Roque <glaroque@baylibre.com>