update contiki-conf to avoid redfintion of UIP_CONF_RECEIVE_WINDOW
update contiki-conf to avoid redfintion of UIP_CONF_RECEIVE_WINDOW

in case of compiling any example that contains the definition of UIP_CONF_RECEIVE_WINDOW over Z1 mote.
i receive this warning: 
 In file included from ../../../core/./contiki.h:38:0,
                   from ../../../core/./contiki-net.h:37,
                   from httpd-simple.c:43:
  ../../../platform/z1/./contiki-conf.h:225:0: warning: "UIP_CONF_RECEIVE_WINDOW" redefined
 ./project-conf.h:60:0: note: this is the location of the previous definition
arurke/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki