package: remove the trailing slash sign from <PKG>_SITE variable

Since the trailing slash is stripped from $($(PKG)_SITE) by pkg-generic.mk:

$(call DOWNLOAD,$($(PKG)_SITE:/=)/$($(PKG)_SOURCE))

so it is redundant.
This patch removes it from $(PKG)_SITE variable for BR consistency.

Signed-off-by: Jerzy Grzegorek <jerzy.grzegorek@trzebnica.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>