BUILDING.md: add osusergo for static build
BUILDING.md: add osusergo for static build

Go 1.11 includes a fix to os/user to be working in a static binary
(fixing https://github.com/golang/go/issues/23265). The fix requires
`osusergo` build tag to be set for static binaries, which is what
this commit documents.

[v2: sort tags alphabetically]

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

containerd/containerd,kunalkushwaha/containerd,estesp/containerd,dmcgowan/containerd,dmcgowan/containerd,crosbymichael/containerd,vdemeester/containerd,thaJeztah/containerd,mikebrow/containerd,miaoyq/containerd,vdemeester/containerd,crosbymichael/containerd,estesp/containerd,miaoyq/containerd,mikebrow/containerd,kunalkushwaha/containerd,containerd/containerd,thaJeztah/containerd