autoconf: single-step includes

Still descend, but only once: we get a lot of mileage out of nodist_

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13316

include/os/linux/spl/sys/Makefile.am

@@ -1,64 +0,0 @@
-KERNEL_H = \
-	acl.h \
-	atomic.h \
-	byteorder.h \
-	callb.h \
-	callo.h \
-	cmn_err.h \
-	condvar.h \
-	cred.h \
-	ctype.h \
-	debug.h \
-	disp.h \
-	dkio.h \
-	errno.h \
-	fcntl.h \
-	file.h \
-	inttypes.h \
-	isa_defs.h \
-	kmem_cache.h \
-	kmem.h \
-	kstat.h \
-	list.h \
-	mod_os.h \
-	mutex.h \
-	param.h \
-	processor.h \
-	proc.h \
-	procfs_list.h \
-	random.h \
-	rwlock.h \
-	shrinker.h \
-	sid.h \
-	signal.h \
-	simd.h \
-	stat.h \
-	string.h \
-	sunddi.h \
-	sysmacros.h \
-	systeminfo.h \
-	taskq.h \
-	thread.h \
-	time.h \
-	timer.h \
-	trace.h \
-	trace_spl.h \
-	trace_taskq.h \
-	tsd.h \
-	types32.h \
-	types.h \
-	uio.h \
-	user.h \
-	vfs.h \
-	vmem.h \
-	vmsystm.h \
-	vnode.h \
-	wait.h \
-	wmsum.h \
-	zmod.h \
-	zone.h
-
-if CONFIG_KERNEL
-kerneldir = @prefix@/src/zfs-$(VERSION)/include/spl/sys
-kernel_HEADERS = $(KERNEL_H)
-endif