pkg-download.mk: do not add a trailing slash to <PKG>_SITE

pkg-download.mk: do not add a trailing slash to <PKG>_SITE

Fixes
http://autobuild.buildroot.net/results/734/7342b25f01d60cafc4a5422a5a1c282629f361c5/

$(dir does not strip last path separator and the wget wrapper adds a '/'
between the -u argument and the filename when it reconstructs the URL,
leading us to have two slashes before the filename when accessing the
upstream location.

Most upstream servers ignores this, but not all - Leading to download issues
from pypi.

Notice: As pointed out by Arnout, we cannot simply use $(PKG)_SITE here as
_PATCH and _EXTRA_DOWNLOADS may contain full URLs.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>