Fix visibility issue with gcc.
Fix visibility issue with gcc.

arvidsson/entityx,Bablawn3d5/entityx,jaynus/entityx,Bablawn3d5/entityx,alecthomas/entityx,drewet/entityx,jarrettchisholm/entityx,Bablawn3d5/entityx,arvidsson/entityx,arvidsson/entityx,jarrettchisholm/entityx,jaynus/entityx,drewet/entityx,jaynus/entityx,jarrettchisholm/entityx,alecthomas/entityx,drewet/entityx,alecthomas/entityx