Add missing help2man build-dep

light.spec.in

 Summary:    Command line tool to control backlight controllers
 URL:        https://github.com/haikarainen/light
 
-License:    GPLv3
-Source0:   __APP_URLBASE__/archive/%{version}.tar.gz
+License:        GPLv3
+Source0:        __APP_URLBASE__/archive/%{version}.tar.gz
+BuildRequires:  help2man
 
 %description
 Light is a program to control backlight controllers under GNU/Linux.