fix header file include errors when compile integration_tests/main.cpp

[vacing]

[CasparKielwein]

I'm willing to merge it.

But I wonder why the old state does not build in your setup. Are you building from an IDE? cd build && cmake .. && make dummy_executable works for me.

[vacing]

I didn't use an IDE, but cmake. I think maybe you install adobe/forest.hpp in your include path, so you can make it on your own computer. It seems there is no question that something is wrong. The solution may be we modify cmake to put adobe/ on the root include path (this may also avoid file conflicts if we have anohter project use adobe/forest.hpp), or adopt the patch what I submitted.