Remove build/ from .gitignore

This exclusion is a leftover from rombert's attempt to implement builds
using PHing (https://www.phing.info/), which was removed by grangeway
back in 2010, during 1.3 development [1].

.gitignore was never updated to reflect this change, and this now
conflicts with our usage of this directory to store build scripts.

[1] see commit 3ea22ce73928309abfb475f0464f974b149ec800