Move VA_COPY macro into compat header.

Some AIX compilers unconditionally undefine va_copy but don't set it back
to an internal function, causing link errors.  In some compat code we
already use VA_COPY instead so move the two existing instances into the
shared header and use for sshbuf-getput-basic.c too.  Should fix building
with at lease some versions of AIX's compiler.  bz#2589, ok djm@

sshbuf-getput-basic.c

@@ -270,7 +270,7 @@ sshbuf_putfv(struct sshbuf *buf, const char *fmt, va_list ap)
 	int r, len;
 	u_char *p;
 
-	va_copy(ap2, ap);
+	VA_COPY(ap2, ap);
 	if ((len = vsnprintf(NULL, 0, fmt, ap2)) < 0) {
 		r = SSH_ERR_INVALID_ARGUMENT;
 		goto out;
@@ -280,7 +280,7 @@ sshbuf_putfv(struct sshbuf *buf, const char *fmt, va_list ap)
 		goto out; /* Nothing to do */
 	}
 	va_end(ap2);
-	va_copy(ap2, ap);
+	VA_COPY(ap2, ap);
 	if ((r = sshbuf_reserve(buf, (size_t)len + 1, &p)) < 0)
 		goto out;
 	if ((r = vsnprintf((char *)p, len + 1, fmt, ap2)) != len) {