Update makefile to use order-only preq for obj directory.
Update makefile to use order-only preq for obj directory.

Prevents rebuilding of all object files when any object file is
modified (since dir timestamp updated and is a dependency of all objects).

maxeler/High-Speed-Packet-Capture,maxeler/High-Speed-Packet-Capture,maxeler/High-Speed-Packet-Capture,maxeler/High-Speed-Packet-Capture