QmlDevTools: Define QML export macros as empty.

QmlDevTools: Define QML export macros as empty.

Since QmlDevTools is a static library, no exports
(Q_QML_EXPORT/Q_QML_PRIVATE_EXPORT) should be specified.

Fixes link errors in qmlimportscanner and lupdate
with MSVC2015:

Qt5QmlDevToolsd.lib(qv4value.obj) : error LNK2019: unresolved external symbol "__declspec(dllimport) public: struct QV4::VTable * __cdecl QV4::Heap::Base::vtable(void)const " (__imp_?vtable@Base@Heap@QV4@@QEBAPEAUVTable@3@XZ) referenced in function "public: bool __cdecl QV4::Managed::isEqualTo(struct QV4::Managed const *)const " (?isEqualTo@Managed@QV4@@QEBA_NPEBU12@@Z)
D:\dev\5-vs15-56\qt-56\qtbase\bin\qmlimportscanner.exe : fatal error LNK1120: 1 unresolved externals

Change-Id: I6125b3d1eaacadba65a90047867779252dab6fbc
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>