Update the compile instructions

[mmahmoudian]

The compile instructions should be updated in this page:

https://flameshot.org/docs/installation/source-code/

The best place to get the required flags is the Github Actions of the main repo.

When done, send the link to the following thread:

flameshot-org/flameshot#4191

[mmahmoudian]

@bhumijain19 thank you for picking this up. I assumed it to you. Let me know if you needed any help.