Update .gitignore

[tgreenx]

Purpose

This PR proposes to align this repository with other Zonemaster components by updating the .gitignore file so that dist files are ignored, i.e. when doing a git status.

Context

N/A

Changes

• Ignore any file starting with "Zonemaster-" in .gitignore

How to test this PR

Before this PR (requires building the dist file first) :

$ git status
On branch develop
Your branch is up to date with 'upstream/develop'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        Zonemaster-LDNS-5.0.1.tar.gz

And after:

$ git status
On branch update-gitignore
nothing to commit, working tree clean

[marc-vanderwal]

I wonder if we still need to ignore Net-LDNS-*.tar.gz.