kbuild: mkcompile_h: trivial cleanups

UTS_TRUNCATTE is simpler this way, and now editors idetify this as a
shell script.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>