package/libjpeg: fix LIBJPEG_SITE

Commit b83184de (package/libjpeg: switch to s.b.o. as source site)
improperly added a trailing slash '/' at the end of LIBJPEG_SITE,
causing builds to fail:

    package/libjpeg/libjpeg.mk:35: *** LIBJPEG_SITE (http://sources.buildroot.org/libjpeg/

) cannot have a trailing slash.  Stop.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>