Make MarginContainer available with disable_advanced_gui=yes

[Calinou]

Comparatively more advanced GUI nodes such as FlowContainer and AspectRatioContainer are already available when building with disable_advanced_gui=yes.

This increases binary size by about 8 KB for binaries compiled with disable_advanced_gui=yes (Linux release export template with LTO enabled).

For context, I'm working on a PR to display license notices in a GUI (similar to #51028). Having MarginContainer available in all situations would help a lot, so that the GUI can keep working in binaries compiled with disable_advanced_gui=yes.

# Current `master` branch when building with `disable_advanced_gui=yes`
60,355,504 godot.linuxbsd.template_release.x86_64.disable_advanced_gui_master

# MarginContainer enabled (what this PR does)
60,363,728 godot.linuxbsd.template_release.x86_64.disable_advanced_gui_pr_margincontainer_only

# MarginContainer, SplitContainer and SubViewportContainer enabled
# (not done in this PR, only included for reference)
60,421,040 godot.linuxbsd.template_release.x86_64.disable_advanced_gui_pr

# `disable_advanced_gui=no` (default)
62,809,040 godot.linuxbsd.template_release.x86_64.enable_advanced_gui

[YuriSizov]

Yeah, this is a weird exclusion, given what is still included.

[YuriSizov]

Thanks!