acl-plugin: acl-as-a-service: VPP-1248: fix the error if exports.h included in more...

acl-plugin: acl-as-a-service: VPP-1248: fix the error if exports.h included in more than one C file

Including the exports.h from multiple .c files belonging to a single plugin results in an error.

Fix that by making all the exported function pointers static, so every place
which includes the exports.h gets its own copy of function pointers.
This will also require to call separately the acl_plugin_exports_init() per file
to initialize them.

Change-Id: Icb70695efa23579c46c716944838766cebc8573e
Signed-off-by: Andrew Yourtchenko <[email protected]>