package/yajl: explicitly request a tag in _SITE

The yajl repository has both a branch and a tag names 2.1.0. Because of
this, when we use the usual github URL [1], github doesn't return a
tarball but rather an ASCII string:
   the given path has multiple possibilities: #<Git::Ref:0x00007f193bebe4d8>, #<Git::Ref:0x00007f193bebd858>

This obviously (and fortunately) leads to a hash check failure.

To make sure we get the tag and not the branch, we should use a URL that
explicitly asks for refs/tags/2.1.0 [2]. We can't change VERSION to
include refs/tags, because that also affects the _SOURCE default, the
contents of the tarball, the CPE ID and the release-monitoring access.
Therefore, add the refs/tags/ prefix to the github helper call.

[1] https://github.com/lloyd/yajl/archive/2.1.0/yajl-2.1.0.tar.gz
[1] https://github.com/lloyd/yajl/archive/refs/tags/2.1.0/yajl-2.1.0.tar.gz



Signed-off-by: Arnout Vandecappelle <arnout@mind.be>