Update IRRLoader.cpp

fix case-sensitive typo.

code/Irr/IRRLoader.cpp

@@ -49,7 +49,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #ifndef ASSIMP_BUILD_NO_IRR_IMPORTER
 
-#include "IRR/IRRLoader.h"
+#include "Irr/IRRLoader.h"
 #include "Common/Importer.h"
 
 #include <assimp/ParsingUtils.h>