That header is not public by default (i.e., it is considered a private
implementation detail of the wifi code) but you can easily make it
public by modifying src/devices/wifi/wscript:
headers = bld.create_obj('ns3header')
headers.module = 'wifi'
headers.source = [
'propagation-delay-model.h'
'wifi-mac-header.h'
...
]
and then, re-running waf which will 'install' the file, that is, copy
this file in build/debug/ns3/
hope this helps,
Mathieu