textproc/askgit: Unbreak build with git2go

Whilst building git2go, it is necessary such that we have
pkg-config installed. Prior to this update, we would fail
building as indicated by:

pkg-config --cflags -- libgit2
pkg-config: exec: "pkg-config": executable file not found in $PATH

Remedy this behaviour by adding USES= pkgconfig.

Reported by: pkg-fallout