package/doxygen: fix _SITE download URL

Doxygen source archive download fails with a HTTP 404 Not Found
error.

The doxygen download page [1] mention that older versions can be
downloaded on SourceForge at [2].

This commit fixes the issue by changing _SITE to this new URL.

Fixes:
https://autobuild.buildroot.org/results/253450bf374ce858fe12712059c435fc773d52f8

[1] https://www.doxygen.nl/download.html
[2] https://sourceforge.net/projects/doxygen/files/

Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/doxygen/doxygen.mk

@@ -6,7 +6,7 @@
 
 DOXYGEN_VERSION = 1.8.18
 DOXYGEN_SOURCE = doxygen-$(DOXYGEN_VERSION).src.tar.gz
-DOXYGEN_SITE = http://doxygen.nl/files
+DOXYGEN_SITE = https://sourceforge.net/projects/doxygen/files/rel-$(DOXYGEN_VERSION)
 DOXYGEN_LICENSE = GPL-2.0
 DOXYGEN_LICENSE_FILES = LICENSE
 DOXYGEN_CPE_ID_VENDOR = doxygen