fix for https://github.com/assimp/assimp/issues/887: add missing define.

code/IRRMeshLoader.h

@@ -48,6 +48,8 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include "BaseImporter.h"
 #include "IRRShared.h"
 
+#ifndef ASSIMP_BUILD_NO_IRRMESH_IMPORTER
+
 namespace Assimp    {
 
 // ---------------------------------------------------------------------------
@@ -92,4 +94,6 @@ protected:
 
 } // end of namespace Assimp
 
+#endif // ASSIMP_BUILD_NO_IRRMESH_IMPORTER
+
 #endif // AI_IRRMESHIMPORTER_H_INC