Fix a -Wswitch-default warning in GCC.

The default case should never actually be met. I've added an assert(false) statement here for sanity.

glm/gtc/quaternion.inl

@@ -707,6 +707,10 @@ namespace detail
 			Result.y = (m[1][2] + m[2][1]) * mult;
 			Result.z = biggestVal;
 			break;
+			
+        default:                // Silence a -Wswitch-default warning in GCC. Should never actually get here. Assert is just for sanity.
+            assert(false);
+            break;
 		}
 		return Result;
 	}