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

[zirias]

Summary

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.

Test Plan

• make makesum in a port with USE_GITLAB does not show a dev warning
• adding MASTER_SITES to that port, both make makesum and make fetch show the warning

[tcberner]

looks sane enough as a work-around :)