misc: Add .gitignore file

The `git status -u` shows the whole content of the 'build' directory
and the the top directories of the `subprojects` directory, making
it too easy to overlook some files not being part of the git checkout,
when they are supposed to be there.
2 jobs for wip/mcrha/add-gitignore in 4 minutes and 8 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #905166
debian-stable-x86_64

00:01:08

passed #905165
fedora-x86_64

00:04:08