windows compilation fix

include/mitsuba/hw/device.h

@@ -115,7 +115,7 @@ private:
 
 /** \brief Abstract device event callback
  */
-class DeviceEventListener {
+class MTS_EXPORT_HW DeviceEventListener {
 public:
 	/** \brief Called when a device event occurs
 	 * @param pEvent The event data structure

include/mitsuba/hw/viewer.h

@@ -31,7 +31,7 @@ MTS_NAMESPACE_BEGIN
  * This class makes it possible to rapidly prototype simple OpenGL
  * applications, which can be started using the 'mtsutil' launcher.
  */
-class Viewer : public Utility, public DeviceEventListener {
+class MTS_EXPORT_HW Viewer : public Utility, public DeviceEventListener {
 public:
 	/// Construct a new viewer
 	Viewer();