Pre-emptively fix games/spring before CMake 3.12 arrives.

file(GLOB .. FOLLOW_SYMLINKS ..) was never documented to work; in
3.12-rc1 this has become an error. That in itself is considered
a regression in CMake [1], but the use is wrong anyway, so patch
it away. The change has been accepted upstream [2] already.

Not bumping PORTREVISION because no code actually changes,
it just removes a cmake-time no-op.

[1] https://gitlab.kitware.com/cmake/cmake/issues/18097
[2] https://springrts.com/mantis/view.php?id=6005

PR: 229101
Reviewed by: tcberner
Differential Revision: https://reviews.freebsd.org/D15878