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://golang.org/cl/122300043

src/cmd/dist/buf.c

@@ -202,7 +202,7 @@ vadd(Vec *v, char *p)
 }
 
 // vaddn adds a string consisting of the n bytes at p to the vector.
-void
+static void
 vaddn(Vec *v, char *p, int n)
 {
 	char *q;