fox build: add missing include for ai_assert.

Signed-off-by: Kim Kulling <[email protected]>

code/OpenGEXImporter.cpp

@@ -45,6 +45,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include <openddlparser/OpenDDLParser.h>
 #include <assimp/scene.h>
+#include <assimp/ai_assert.h>
 
 #include <vector>