support/script/scancpan: don't generate the trailing slash in <PKG>_SITE variable

The trailing slash was removed in all packages, so don't generate it.
See http://git.buildroot.net/buildroot/commit/?id=c7f4b964718bc5a3329bb730f59e24091ecbc7a2

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>