[package] fix vnstat install usage on FreeBSD and MacOSX (#7718)

[package] fix vnstat install usage on FreeBSD and MacOSX (#7718)

vnstat's makefile uses the '-D' option which is only part of GNU install,
thus compiling vnstat for OpenWrt on FreeBSD or Mac OS X fails because of
the missing option.

Attached is a patch to have the install target separately create the
destination directory before installing the vnstat.conf file to $(DEST)/etc.
Additionally it also removes an erroneous tab character in the Makefile.

The patch is meant to go in feeds/packages/net/vnstat/patches/ directory.

TRAC Ticket: https://dev.openwrt.org/attachment/ticket/7718/

Signed-off-by: Joe Roback <joe@roback.cc>
git-svn-id: svn://svn.openwrt.org/openwrt/packages@22913 3c298f89-4303-0410-b956-a3cf2f4a3e73