fix declaration of symbol of different size

fix declaration of symbol of different size

I got many warnings at the link step with gcc version 5.3.1 20160225 (Ubuntu 5.3.1-10ubuntu2):

/usr/bin/ld: Warning: size of symbol `cop_input_node' changed from 112 in vnet/cop/.libs/cop.o to 168 in vnet/cop/.libs/node1.o
/usr/bin/ld: Warning: size of symbol `ethernet_input_node' changed from 112 in vnet/.libs/interface.o to 136 in vnet/ethernet/.libs/node.o
/usr/bin/ld: Warning: size of symbol `l2output_node' changed from 112 in vnet/l2/.libs/l2_efp_filter.o to 120 in vnet/l2/.libs/l2_output.o
/usr/bin/ld: Warning: size of symbol `srp_input_node' changed from 112 in vnet/srp/.libs/format.o to 136 in vnet/srp/.libs/node.o
/usr/bin/ld: Warning: size of symbol `vxlan_encap_node' changed from 112 in vnet/vxlan/.libs/vxlan.o to 128 in vnet/vxlan/.libs/encap.o
/usr/bin/ld: Warning: size of symbol `vxlan_input_node' changed from 112 in vnet/vxlan/.libs/vxlan.o to 144 in vnet/vxlan/.libs/decap.o
...

Looking at vlib_node_registration_t, I think the reason is that
the char * next_nodes[] could be bigger where the variable is defined
in .c file.

We should mark global variables as external in header files.

Some of them can be made static.

Change-Id: Ieb6961fd08180c9a69e1d884852703f3eb23f23f
Signed-off-by: Jean-Mickael Guerin <jean-mickael.guerin@6wind.com>