deskutils/subsurface: Fix build on older FreeBSD versions

On FreeBSD 12.3, build fail with:

In file included from /wrkdirs/usr/ports/deskutils/subsurface/work/subsurface-5.0.10/core/exif.cpp:32:
/wrkdirs/usr/ports/deskutils/subsurface/work/subsurface-5.0.10/core/exif.h:128:3: error: unknown type name 'time_t'

time_t epoch();
      ^

1 error generated.

Explicitly add time.h to headers which use time_t to avoid the issue.
This should not have an impact where the package could be build so not
bumping PORTREVISION.