Fixes:

build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/linux-mvebu_cortexa9/gpio-button-hotplug/gpio-button-hotplug.c:193:2: error: implicit declaration of function 'broadcast_uevent'; did you mean 'kobject_uevent'? [-Werror=implicit-function-declaration]
  broadcast_uevent(event->skb, 0, 1, GFP_KERNEL);
  ^~~~~~~~~~~~~~~~
  kobject_uevent
cc1: some warnings being treated as errors