$ dpkg -L libsoil-dev |grep include
/usr/include
/usr/include/SOIL
/usr/include/SOIL/SOIL.h
/usr/include/SOIL/image_DXT.h
/usr/include/SOIL/image_helper.h
/usr/include/SOIL/stbi_DDS_aug.h
/usr/include/SOIL/stbi_DDS_aug_c.h
/usr/include/SOIL/stb_image_aug.h
So you probably want the following on the g++ command-line
-I /usr/include/SOIL
Or just use the following in your C++
#include <SOIL/SOIL.h>
And you probably want the following on the command-line when linking
-lSOIL