Fix compilation on Windows

There is a vast amount of linkage warnings for glraw classes and especially this one is an error which prevents building glraw:
```
D:\jenkins\workspace\glraw-windows-msvc2015\source\glraw\source\RawFile.cpp(45): error C2491: 'glraw::RawFile::s_signature': definition of dllimport static data member not allowed [D:\jenkins\workspace\glraw-windows-msvc2015\build\source\glraw\glraw.vcxproj]
```