u8_textprep: move into module/zfs

Now that it's built into the main zfs module in all cases, there's no
reason to put it in its own dir.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #18071

lib/libzpool/Makefile.am

@@ -46,8 +46,6 @@ nodist_libzpool_la_SOURCES = \
 	\
 	module/os/linux/zfs/zio_crypt.c \
 	\
-	module/unicode/u8_textprep.c \
-	\
 	module/zcommon/cityhash.c \
 	module/zcommon/simd_stat.c \
 	module/zcommon/zfeature_common.c \
@@ -137,6 +135,7 @@ nodist_libzpool_la_SOURCES = \
 	module/zfs/space_map.c \
 	module/zfs/space_reftree.c \
 	module/zfs/txg.c \
+	module/zfs/u8_textprep.c \
 	module/zfs/uberblock.c \
 	module/zfs/unique.c \
 	module/zfs/vdev.c \