Fix building the OpenVG graphicssystem on Linux with static libs

Fix building the OpenVG graphicssystem on Linux with static libs

Without this, building OpenVG would fail because Q_OPENVG_EXPORT wasn't
defined at all, causing it to be treated as a variable name

Merge-request: 1256
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
(cherry picked from commit 8680ced782c5e225b2e15c50c05493a23410b119)

Change-Id: I0da36af8cdcf411325fad25fa5c4d2ab7764c0ee
Reviewed-on: http://codereview.qt-project.org/4573
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>