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/kernel/Makefile.am

@@ -1 +0,0 @@
-SUBDIRS = linux

include/os/linux/kernel/linux/Makefile.am

@@ -1,22 +0,0 @@
-KERNEL_H = \
-	dcache_compat.h \
-	xattr_compat.h \
-	vfs_compat.h \
-	blkdev_compat.h \
-	utsname_compat.h \
-	kmap_compat.h \
-	percpu_compat.h \
-	simd.h \
-	simd_x86.h \
-	simd_aarch64.h \
-	simd_powerpc.h \
-	mod_compat.h \
-	page_compat.h \
-	compiler_compat.h
-
-if CONFIG_KERNEL
-if BUILD_LINUX
-kerneldir = @prefix@/src/zfs-$(VERSION)/include/linux
-kernel_HEADERS = $(KERNEL_H)
-endif
-endif