Fixed build error

 [Problem] Build break occurred with the following error message:
            mac/mac-802_11Ext.h:175:19: error: expected primary-expression before ‘struct’
            mac/mac-802_11Ext.h:175:41: error: ‘dh_body’ was not declared in this scope
            mac/mac-802_11Ext.h:175:51: error: ‘offsetof’ was not declared in this scope
[Solution] Added <stddef.h> in ns-2.34/mac/mac-802_11Ext.h