VF-84 Jolly Roger style design for the HGU-68 Helmet

VF-84 Jolly Roger style design for the HGU-68 Helmet.

Thanks to Chewmann for the help on getting the skin ingame.

Add this line in the LUA files in the Pilot Maps

    {"pilot_F18_helmet_HGU68", 0 ,"pilot_F18_helmet",false};
    {"pilot_F18_helmet_HGU68", ROUGHNESS_METALLIC ,"pilot_f18_helmet_roughmet",false};
    {"pilot_F18_helmet_HGU68", 1 ,"pilot_F18_helmet_nm",false};