pjproject: bump to 2.13.1

pjproject: bump to 2.13.1

1. add "/refs/tags" to PKG_SOURCE_URL, otherwise the downloaded file is
   broken now
2. bump minor version, includes security related fixes, see [1], [2] and
   [3]

[1] https://github.com/pjsip/pjproject/security/advisories/GHSA-9pfh-r8x4-w26w
[2] https://github.com/pjsip/pjproject/security/advisories/GHSA-cxwq-5g9x-x7fr
[3] https://github.com/pjsip/pjproject/security/advisories/GHSA-q9cp-8wcq-7pfr

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>
(cherry picked from commit 73526e5661a3c357edeeeac407a7560b9d485a54)