#include glm/gtc/matrix_transform.hpp in SceneGraph.h to fix compile error

All the GLM matrix transformation calls in SceneGraph.h were giving "function does not exist" errors. Turns out, they're *not* in glm/glm.hpp like we thought - they're actually in glm/gtc/matrix_transform.hpp. Because this header was being included in Chair.h, it was by extension included in MyGLWidget.cpp, which is why we were able to access the transformation functions there without having it explicitly included.