Add missing files to Makefile

Some .h files that were added were missed in this Makefile. Since 
they are .h files, their being missing only resulted in them 
disappeared from the dist archive.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Closes #11705

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

@@ -4,6 +4,7 @@ KERNEL_H = \
 	atomic.h \
 	byteorder.h \
 	callb.h \
+	ccompat.h \
 	ccompile.h \
 	cmn_err.h \
 	condvar.h \
@@ -18,9 +19,11 @@ KERNEL_H = \
 	fcntl.h \
 	file.h \
 	freebsd_rwlock.h \
+	idmap.h \
 	inttypes.h \
 	isa_defs.h \
 	kmem_cache.h \
+	kidmap.h \
 	kmem.h \
 	kstat.h \
 	list_impl.h \