lib: hide internal prefix list structures

These are about to be touched and there's no point in other code
touching into prefix list's internas.  Add some isolation.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/lib/Makefile.am b/lib/Makefile.am
index 56837d8..fc02312 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -30,6 +30,9 @@
 	privs.h sigevent.h pqueue.h jhash.h zassert.h memtypes.h \
 	workqueue.h route_types.h libospf.h
 
+noinst_HEADERS = \
+	plist_int.h
+
 EXTRA_DIST = \
 	regex.c regex-gnu.h \
 	queue.h \