Fix build using FreeBSD clang version 6.0.1 (#2459)
Fix build using FreeBSD clang version 6.0.1 (#2459)

[ 19%] Building CXX object Source/ThirdParty/SLikeNet/CMakeFiles/SLikeNet.dir/Source/src/BitStream.cpp.o
In file included from /home/romain/Projects/Urho3D/Source/ThirdParty/SLikeNet/Source/src/BitStream.cpp:48:
/home/romain/Projects/Urho3D/Source/ThirdParty/SLikeNet/Source/include/slikenet/linux_adapter.h:17:10: fatal error: 'winsock2.h' file not found
#include "winsock2.h"
         ^~~~~~~~~~~~
1 error generated.
orefkov/Urho3D,weitjong/Urho3D,SirNate0/Urho3D,weitjong/Urho3D,henu/Urho3D,rokups/Urho3D,eugeneko/Urho3D,rokups/Urho3D,rokups/Urho3D,orefkov/Urho3D,eugeneko/Urho3D,rokups/Urho3D,SirNate0/Urho3D,urho3d/Urho3D,SuperWangKai/Urho3D,eugeneko/Urho3D,bacsmar/Urho3D,SuperWangKai/Urho3D,urho3d/Urho3D,urho3d/Urho3D,SuperWangKai/Urho3D,eugeneko/Urho3D,henu/Urho3D,henu/Urho3D,bacsmar/Urho3D,urho3d/Urho3D,PredatorMF/Urho3D,SirNate0/Urho3D,bacsmar/Urho3D,rokups/Urho3D,bacsmar/Urho3D,rokups/Urho3D,henu/Urho3D,PredatorMF/Urho3D,SuperWangKai/Urho3D,PredatorMF/Urho3D,weitjong/Urho3D,weitjong/Urho3D,henu/Urho3D,weitjong/Urho3D,orefkov/Urho3D,SirNate0/Urho3D,PredatorMF/Urho3D,SuperWangKai/Urho3D,orefkov/Urho3D,SirNate0/Urho3D