package/szip: update SZIP_SITE

Fix broken link.

While at it, use $(SZIP_VERSION) inside SZIP_SITE instead of
duplicating the version.

Fixes:

 http://autobuild.buildroot.org/results/35d/35d08103bbd0da22a5e111f1f86f28aba7062ee6

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/szip/szip.mk

@@ -5,7 +5,7 @@
 ################################################################################
 
 SZIP_VERSION = 2.1.1
-SZIP_SITE = http://www.hdfgroup.org/ftp/lib-external/szip/2.1.1/src
+SZIP_SITE = https://docs.hdfgroup.org/archive/support/ftp/lib-external/szip/$(SZIP_VERSION)/src
 SZIP_LICENSE = szip license
 SZIP_LICENSE_FILES = COPYING
 SZIP_INSTALL_STAGING = YES