Updating ns3 lib flag compilation

Change NS3_LIB to NS3_OFSWITCH (to use same flag from ns3), and
“unhide” some static methods from source, exposing them in headers only
when library.
Fixing wrong #ifdef command.
Revert "Moving dp_buffers strucutres to header file"

This reverts commit 49532b2db601c389e671ae8a735e0ded9417e3f8.
Allowing ns3 to use dpctl parse methods.