package/rp-pppoe: fixup download URL

As reported in [1], the download of rp-pppoe-3.15.tar.gz is no longer
guaranteed from the download section of the project.

Fortunately, Jacon Kroon is hosting the RP-PPPoE source code for versions
3.15 and 4.0 in a location that permits automated downloads.

Fixes:
- http://autobuild.buildroot.org/results/8676456feec9dd401152887101ad1a9ce96cb769

[1] https://dianne.skoll.ca/pipermail/rp-pppoe/2024q3/000652.html

Cc: Jaco Kroon <jaco@uls.co.za>
Cc: Dianne Skoll <dianne@skoll.ca>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/rp-pppoe/rp-pppoe.mk

@@ -5,7 +5,7 @@
 ################################################################################
 
 RP_PPPOE_VERSION = 3.15
-RP_PPPOE_SITE = https://dianne.skoll.ca/projects/rp-pppoe/download
+RP_PPPOE_SITE = https://downloads.uls.co.za/rp-pppoe
 RP_PPPOE_LICENSE = GPL-2.0
 RP_PPPOE_LICENSE_FILES = doc/LICENSE
 RP_PPPOE_CPE_ID_VALID = YES