bsd.sites.mk: Avoid bogus dev-warning from makesum

makesum calls make fetch and passes MASTER_SITES, so the dev warning
about it is triggered by the value USE_GITLAB computes. Avoid this by
disabling the warning in make fetch when NO_CHECKSUM and DISABLE_SIZE
are set.

Approved by: portmgr (tcberner, mentor)
Differential Revision: https://reviews.freebsd.org/D41014