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/ospfd/ospf_zebra.c b/ospfd/ospf_zebra.c
index b6d3260..bdb5193 100644
--- a/ospfd/ospf_zebra.c
+++ b/ospfd/ospf_zebra.c
@@ -1126,7 +1126,7 @@
     {
       /* Update filter-list in. */
       if (PREFIX_NAME_IN (area))
-        if (strcmp (PREFIX_NAME_IN (area), plist->name) == 0)
+        if (strcmp (PREFIX_NAME_IN (area), prefix_list_name (plist)) == 0)
           {
             PREFIX_LIST_IN (area) =
               prefix_list_lookup (AFI_IP, PREFIX_NAME_IN (area));
@@ -1135,7 +1135,7 @@
 
       /* Update filter-list out. */
       if (PREFIX_NAME_OUT (area))
-        if (strcmp (PREFIX_NAME_OUT (area), plist->name) == 0)
+        if (strcmp (PREFIX_NAME_OUT (area), prefix_list_name (plist)) == 0)
           {
             PREFIX_LIST_IN (area) =
               prefix_list_lookup (AFI_IP, PREFIX_NAME_OUT (area));