package/boost: fix broken BOOST_SITE URL

Current URL returns 403 error:

--2021-05-10 10:04:12--  https://dl.bintray.com/boostorg/release/1.75.0/source/boost_1_75_0.tar.bz2
Resolving dl.bintray.com... 18.193.131.58, 3.66.199.110
Connecting to dl.bintray.com|18.193.131.58|:443... connected.
HTTP request sent, awaiting response... 403 Forbidden
2021-05-10 10:04:12 ERROR 403: Forbidden.

Bintray has been sunset on May 1st:
https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/

Update the URL to the new upstream location to fix this issue.

Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/boost/boost.mk

@@ -6,7 +6,7 @@
 
 BOOST_VERSION = 1.75.0
 BOOST_SOURCE = boost_$(subst .,_,$(BOOST_VERSION)).tar.bz2
-BOOST_SITE = https://dl.bintray.com/boostorg/release/$(BOOST_VERSION)/source
+BOOST_SITE = https://boostorg.jfrog.io/artifactory/main/release/$(BOOST_VERSION)/source
 BOOST_INSTALL_STAGING = YES
 BOOST_LICENSE = BSL-1.0
 BOOST_LICENSE_FILES = LICENSE_1_0.txt