fix dpdk_plugin.so load failure with DPDK 18.08

fix dpdk_plugin.so load failure with DPDK 18.08

load_one_plugin:189: Loaded plugin: abf_plugin.so (ACL based Forwarding)
load_one_plugin:189: Loaded plugin: acl_plugin.so (Access Control Lists)
load_one_plugin:189: Loaded plugin: avf_plugin.so (Intel Adaptive Virtual Function (AVF) Device Plugin)
load_one_plugin:191: Loaded plugin: cdp_plugin.so
load_one_plugin:145: /home/lijian/tasks/_ipsum/vpp/build-root/install-vpp-native/vpp/lib64/vpp_plugins/dpdk_plugin.so: undefined symbol: mnl_socket_close
load_one_plugin:146: Failed to load plugin 'dpdk_plugin.so'
load_one_plugin:189: Loaded plugin: flowprobe_plugin.so (Flow per Packet)
load_one_plugin:189: Loaded plugin: gbp_plugin.so (Group Based Policy)

Change-Id: I4f885f2bd3e548dbb2f7afec3363ed8ae09b3eca
Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>