fix db_notify.cpp not included in disable unity build db set

[ihla]

this PR fixes linker error when GRAPHENE_DISABLE_UNITY_BUILD=true

[oxarbitrage]

thank you for fixing this. i confirmed the project will not build when GRAPHENE_DISABLE_UNITY_BUILD is true and adding db_notify.cpp alone is not enough; it needs the includes in this pull request as well to compile.

i am not really sure why someone will disable the feature but thats another thing(please excuse my ignorance in the subject).

when enabled(default) project compiles and run fine too so the changes will not affect the default build and fix the problem when flag is used.

thanks again, approving now and will be merged soon.

[ihla]

there is only one reason why I have enabled it - when working with the IDE like Clion one can use many of the convenient IDE features like code navigation and browsing for which the files must be included in the project - the Clion takes only files included in the build from the cmake file, I don't observer any compilation slowing down with GRAPHENE_DISABLE_UNITY_BUILD when running on my notebook with SSD

[oxarbitrage]

interesting, i use clion, i am going to give a try to that.