package/libjpeg: fix LIBJPEG_SITE

Commit b83184de674a (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>
(cherry picked from commit 308f4428c8d8cbe6ea563b295f590a4c3da23646)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libjpeg/libjpeg.mk

@@ -9,7 +9,7 @@ LIBJPEG_VERSION = 9d
 # 2021-03, causing hash mismatch. Until there is a new version released,
 # use our cached copy from s.b.o.
 #LIBJPEG_SITE = http://www.ijg.org/files
-LIBJPEG_SITE = http://sources.buildroot.org/libjpeg/
+LIBJPEG_SITE = http://sources.buildroot.org/libjpeg
 LIBJPEG_SOURCE = jpegsrc.v$(LIBJPEG_VERSION).tar.gz
 LIBJPEG_LICENSE = IJG
 LIBJPEG_LICENSE_FILES = README