tools: lib: thermal: use pkg-config to locate libnl3

To make libthermal more cross compile friendly use pkg-config to locate
libnl3. Only if that fails fall back to hardcoded /usr/include/libnl3.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>