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/zfs/sys/Makefile.am

@@ -1,31 +0,0 @@
-KERNEL_H = \
-	policy.h \
-	sha2.h \
-	trace_acl.h \
-	trace_arc.h \
-	trace_common.h \
-	trace_zfs.h \
-	trace_dbgmsg.h \
-	trace_dbuf.h \
-	trace_dmu.h \
-	trace_dnode.h \
-	trace_multilist.h \
-	trace_rrwlock.h \
-	trace_txg.h \
-	trace_vdev.h \
-	trace_zil.h \
-	trace_zio.h \
-	trace_zrlock.h \
-	zfs_bootenv_os.h \
-	zfs_context_os.h \
-	zfs_ctldir.h \
-	zfs_dir.h \
-	zfs_vfsops_os.h \
-	zfs_vnops_os.h \
-	zfs_znode_impl.h \
-	zpl.h
-
-if CONFIG_KERNEL
-kerneldir = @prefix@/src/zfs-$(VERSION)/include/sys
-kernel_HEADERS = $(KERNEL_H)
-endif