Try to add
"/usr/include/x86_64-linux-gnu" directory to include paths in /usr/local/pocketbook/common.mk:
ifeq ($(BUILD), emu)
INCLUDES += /usr/include/x86_64-linux-gnu
Probably they've changed the default kernel include files path but the compiler knows nothing about that?
2. Another interesting question:
Why they (PocketBook) need to include the following system headers to incdemo.c?
I've removed them and everything was compiled fine. Probably these headers was from the historical purpose