ip: fix path MTU node errors definition

The path mtu node uses errors defined by ip fragmentation.

Type: fix

Change-Id: I1f173955919a4f555ab0309cd8201ec342a0ae92
Signed-off-by: Benoît Ganne <bganne@cisco.com>
diff --git a/src/vnet/ip/ip_frag.c b/src/vnet/ip/ip_frag.c
index cafa9a6..b9bc90d 100644
--- a/src/vnet/ip/ip_frag.c
+++ b/src/vnet/ip/ip_frag.c
@@ -502,7 +502,7 @@
   return IP_FRAG_ERROR_NONE;
 }
 
-static char *ip4_frag_error_strings[] = {
+char *ip4_frag_error_strings[] = {
 #define _(sym,string) string,
   foreach_ip_frag_error
 #undef _