win32: fix building with NO_UNIX_SOCKETS
After
2406bf5f (Win32: detect unix socket support at runtime,
2024-04-03), it fails with:
compat/mingw.c:4160:5: error: no previous prototype for function 'mingw_have_unix_sockets' [-Werror,-Wmissing-prototypes]
4160 | int mingw_have_unix_sockets(void)
| ^
because the prototype is behind `ifndef NO_UNIX_SOCKETS`.
Signed-off-by: Mike Hommey <mh@glandium.org>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>