need update assimp head file

[beinimaliesi]

should provide libassimp.dylib for macos, because i found use repo assimp head file and /usr/local/lib/libassimp.dylib will cause header file and dylib not same version.

like :

Texture failed to load at path: s_dif.png
Texture failed to load at path: s_ddn.png
Texture failed to load at path: s_refl.png

take long time found this issue, hope for fix it.

[simpzan]

I created a PR to fix this. I tested on macOS and it works.
#374