serve: mark has_capability() as static

serve: mark has_capability() as static

The has_capability() function introduced in ed10cb952d3 (serve:
introduce git-serve, 2018-03-15) has never been used anywhere except
serve.c, so let's mark it as static.

It was later changed from "extern" in 554544276a6 (*.[ch]: remove
extern from function declarations using spatch, 2019-04-29), but we
could have simply marked it as "static" instead.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>