Add build/ dir to .gitignore

[iasdeoupxe]

Running install.sh or following the instructions given in https://github.com/petrockblog/PowerBlock#building-and-installation is leaving an untracked build dir behind which we could hide via the .gitignore:

# git status
On branch master
Your branch is up-to-date with 'origin/master'.
Untracked files:
  (use "git add <file>..." to include in what will be committed)

    build/

nothing added to commit but untracked files present (use "git add" to track)

[iasdeoupxe]

Build failure seems to be completely unrelated.