thermal: Fix compiler warning

The following warning is obtained when CONFIG_NET is not defined:

In file included from drivers/thermal/mvebu_thermal.c:27:0:
include/linux/thermal.h:254:12: warning: 'thermal_generate_netlink_event'
defined but not used [-Wunused-function]

This patch fixes the warning by properly inlining
thermal_generate_netlink_event().

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>