restructuring: windows: search function by symbol in first place, regardless if it exported or not in original HLSDK

[SmileyAG]

There is a fair reason for this: we want modders in the future to support BXT on their modification side (or use the SDKs, authors of those made the support), and therefore it must for modder to export symbols for all functions that BXT uses in the code

Macro for export:

#ifdef _WIN32
#define DLLEXPORT __declspec(dllexport)
#else
#define DLLEXPORT __attribute__ ((visibility("default")))
#endif