patch 9.1.1624: Cscope not enabled on MacOS

patch 9.1.1624: Cscope not enabled on MacOS

Problem:  Cscope not enabled on MacOS
Solution: Remove #ifdef test for MacOS and always enable FEAT_CSCOPE on
          Unix (Damien Lejay).

Remove the !MACOS_X guard from FEAT_CSCOPE in feature.h. macOS has been a
certified UNIX since 10.5 and supports cscope without issues. This guard
originated in the pre-OS X era when Classic MacOS lacked the required
POSIX APIs. MacVim already ships with +cscope successfully.

closes: #17976

Signed-off-by: Damien Lejay <damien@lejay.be>
Signed-off-by: Christian Brabandt <cb@256bit.org>