Move GRE folder under vnet to the plugin folder, and modify some of path
of the #inlude<header> to the new path.
Add a plugin.c file to register a plugin.
JIRA: VPP-2044
Type: improvement
Change-Id: I7f64cecd97538a7492e56a41558dab58281a9fa5
Signed-off-by: Chuhao Tang <nicotang@cisco.com>
