dist: Make vaddn private to buf.c
dist: Make vaddn private to buf.c

This function does not have a declaration/prototype in a.h, and it is used only
in buf.c, so it is local to it and thus can be marked as private by adding
'static' to it.

LGTM=iant
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/122300043

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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