MSVC: Fix 'inconsistent dll linkage' warnings
MSVC: Fix 'inconsistent dll linkage' warnings

Fixes warnings introduced in f489dde33403e43228:

api/projectdata.h(72): warning C4273: 'qbs::operator ==': inconsistent dll linkage
api/projectdata.h(66): note: see previous definition of '=='
api/projectdata.h(73): warning C4273: 'qbs::operator !=': inconsistent dll linkage
api/projectdata.h(67): note: see previous definition of '!='

Change-Id: Iafcb75dc29db1fa5100781ec0a802936c874c621
Reviewed-by: Christian Kandeler <3aa99dcdd3f0cac61fb81c2a11771c0cc2497607@qt.io>

qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs