misc/libpri: update to 1.6.1

ChangeLog: https://www.asterisk.org/asterisk-news/libpri-1-6-1-now-available/

 * .github: Add Releaser workflow
 * Link README to README.md
 * Makefile: Fix modern compiler errors.
 * Makefile: Add the ability to build libpri on MacOS for Linux target.
 * q931.c: Fix subaddress finding octet 4.

Reported by:	portscout