Using Q_NAMESPACE_EXPORT from Qt 5.14 on

[vrcomputing]

Fixes the following unresolved external symbol error when compiling QSkinny on Windows using MSVC 2019 in conjunction with Qt 5.15.2 and MSBuild Tools.

[build]   QskMaterial3SkinFactory.cpp
[build]      Creating library C:/repositories/qskinny_vrcomputing-build/skins/squiek/Debug/squiekskin.lib and object C:/repositories/qskinny_vrcomputing-build/skins/squiek/Debug/squiekskin.exp
[build] QskSquiekSkin.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) struct QMetaObject const Qsk::staticMetaObject" (__imp_?staticMetaObject@Qsk@@3UQMetaObject@@B) referenced in function "struct QMetaObject const * __cdecl Qsk::qt_getEnumMetaObject(enum Qsk::Direction)" (?qt_getEnumMetaObject@Qsk@@YAPEBUQMetaObject@@W4Direction@1@@Z) [C:\repositories\qskinny_vrcomputing-build\skins\squiek\squiekskin.vcxproj]
[build]     Hint on symbols that are defined and could potentially match:
[build]       "__declspec(dllimport) public: static struct QMetaObject const QskSkin::staticMetaObject" (__imp_?staticMetaObject@QskSkin@@2UQMetaObject@@B)
[build]       "__declspec(dllimport) public: static struct QMetaObject const QskSkinFactory::staticMetaObject" (__imp_?staticMetaObject@QskSkinFactory@@2UQMetaObject@@B)
[build] C:\repositories\qskinny_vrcomputing-build\skins\squiek\Debug\squiekskin.dll : fatal error LNK1120: 1 unresolved externals [C:\repositories\qskinny_vrcomputing-build\skins\squiek\squiekskin.vcxproj]
[build]   Generating Code...
[build]      Creating library C:/repositories/qskinny_vrcomputing-build/skins/material3/Debug/material3skin.lib and object C:/repositories/qskinny_vrcomputing-build/skins/material3/Debug/material3skin.exp
[build] QskMaterial3Skin.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) struct QMetaObject const Qsk::staticMetaObject" (__imp_?staticMetaObject@Qsk@@3UQMetaObject@@B) referenced in function "struct QMetaObject const * __cdecl Qsk::qt_getEnumMetaObject(enum Qsk::Direction)" (?qt_getEnumMetaObject@Qsk@@YAPEBUQMetaObject@@W4Direction@1@@Z) [C:\repositories\qskinny_vrcomputing-build\skins\material3\material3skin.vcxproj]
[build]     Hint on symbols that are defined and could potentially match:
[build]       "__declspec(dllimport) public: static struct QMetaObject const QskSkin::staticMetaObject" (__imp_?staticMetaObject@QskSkin@@2UQMetaObject@@B)
[build]       "__declspec(dllimport) public: static struct QMetaObject const QskSkinFactory::staticMetaObject" (__imp_?staticMetaObject@QskSkinFactory@@2UQMetaObject@@B)
[build] C:\repositories\qskinny_vrcomputing-build\skins\material3\Debug\material3skin.dll : fatal error LNK1120: 1 unresolved externals [C:\repositories\qskinny_vrcomputing-build\skins\material3\material3skin.vcxproj]