ao_write: Silence sparse warning.

ao_write: Silence sparse warning.

	ao_write.c:336:58: warning: incorrect type in argument 3 (different modifiers)
	ao_write.c:336:58:    expected void *( *__start_routine )( ... )
	ao_write.c:336:58:    got void [noreturn] *( *<noident> )( ... )

gcc versions 5, 6 and 7 all seem to be happy without the __noreturn
as well, so let's lose it.