sysutils/backuppc, sysutils/backuppc4: Add GitHub to MASTER_SITES

Not using the Github framework on purpose for getting the same release file
hosted on Github and Sourceforge.

PR: 219653
Submitted by: Alexander Moisseev <moiseev@mezonplus.ru> (maintainer)
Approved by: garga (mentor)
Differential Revision: https://reviews.freebsd.org/D11046