pg: Reduce the inclusion of pg.h

Type: style

reduce the number of files recompiled after changing pg.h from 1110 to
102.

Signed-off-by: Neale Ranns <neale@graphiant.com>
Change-Id: I50611eba818eeb3a2dffd437a3c72c77766bed80
diff --git a/src/vnet/arp/arp.c b/src/vnet/arp/arp.c
index 299ed97..ced3c1c 100644
--- a/src/vnet/arp/arp.c
+++ b/src/vnet/arp/arp.c
@@ -22,6 +22,7 @@
 #include <vnet/fib/fib_entry_src.h>
 #include <vnet/adj/adj_nbr.h>
 #include <vnet/adj/adj_mcast.h>
+#include <vnet/pg/pg.h>
 
 #include <vnet/ip-neighbor/ip_neighbor.h>
 #include <vnet/ip-neighbor/ip_neighbor_dp.h>