chapters/compute/lecture-demos: Fix missing Makefile `include`s

[teodutu]

#74 transitioned the Compute chapter to the Open Education Hub methodology, creating the lecture-demos subchapter [1]. The makefiles in these demos (such as [2]) reference an inexisting common Makefile we plan to remove.

These makefiles need to be updated and made independent of other makefiles. The Makefile for each demo must contain all the rules necessary to create the binary files.

[1] https://github.com/cs-pub-ro/operating-systems/tree/bce2316283a7585b23f9adfc54c85c89e3ed32cd/chapters/compute/lecture-demos
[2]

operating-systems/chapters/compute/lecture-demos/copy-on-write/Makefile

Line 3 in bce2316

include ../../../../../common/makefile/single.mk