build/arm: Arm compilation warning fix

The arm cross compiler is issuing warnings for signed/unsigned
comparisons for ntohs.  ntohs returns a unsigned int, while
the counting variables are signed.  Fixed to allow -Werror
to work properly

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
diff --git a/ospfd/ospf_interface.c b/ospfd/ospf_interface.c
index af4f0a6..f41c4f1 100644
--- a/ospfd/ospf_interface.c
+++ b/ospfd/ospf_interface.c
@@ -1011,7 +1011,7 @@
   struct ospf_interface *voi;
   struct listnode *node;
   struct vertex_parent *vp = NULL;
-  int i;
+  unsigned int i;
   struct router_lsa *rl;
 
   voi = vl_data->vl_oi;