brt: lift internal definitions into _impl header

So that zdb (and others!) can get at the BRT on-disk structures.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Kay Pedersen <mail@mkwg.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #15541

include/Makefile.am

@@ -33,6 +33,7 @@ COMMON_H = \
 	sys/bqueue.h \
 	sys/btree.h \
 	sys/brt.h \
+	sys/brt_impl.h \
 	sys/dataset_kstats.h \
 	sys/dbuf.h \
 	sys/ddt.h \