thermal: add missing Kconfig dependency

Otherwise THERMAL_HWMON can be selected when HWMON=n and THERMAL=n, which
fails to build.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>