tweak nav includes

partials/index-comic-post.inc

@@ -1,6 +1,6 @@
 <div class="post-comic-head"></div>
 <div class="post-comic">
-  <?php include('nav.inc') ?>
+  <?php include_partial('nav') ?>
   <div class="comicdate"><?php the_date('F jS, Y') ?></div>
   <br class="clear" />
   <h2>

partials/single-comic-post.inc

@@ -1,7 +1,7 @@
 <div class="post-comic-head"></div>
 <div class="post-comic" id="post-<?php the_ID() ?>">
   <div class="comicdate">
-    <?php include('nav.inc') ?>
+    <?php include_partial('nav') ?>
     <?php the_time('F jS, Y') ?>
   </div>
   <?php if (get_option('comicpress-enable-storyline-support') == 1) { ?>