x11/lightdm-gtk-greeter: fix build with GCC-based architectures

Add USE_CSTD=c99 to fix:
lightdm-gtk-greeter.c: In function 'start_authentication':
lightdm-gtk-greeter.c:2023: error: 'for' loop initial declaration used outside C99 mode

PR: 237626
Approved by: woodsb02 (maintainer), mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20094