fix: use functional interface for GL callback

[saadndm]

Description

Changes the Skiko OpenGL address callback to use a functional interface, fixing OneConfig GUI not initializing on wayland.

MC 26.2 uses org.lwjgl:lwjgl:3.4.1:unsafe, which falls back to libffi callbacks and hides this issue.

But 26.1, https://github.com/moehreag/legacy-lwjgl3 (for Ornithe), and eventually 26.3 all use regular LWJGL builds with FFM upcalls in Java 25.

LWJGL requires the FFM upcall interface to be annotated with @FunctionalInterface, currently causing this error on 26.1 Wayland:

[20:30:02] [Render thread/WARN] (GLInterfaceFactory) GLInterfaceFactory: failed to create getProc closure

java.lang.UnsupportedOperationException: The upcall interface must be annotated with @FunctionalInterface
	at knot//org.lwjgl.system.ffm.BCCallUp.<init>(BCCallUp.java:52)
	at knot//org.lwjgl.system.ffm.FFM.ffmUpcall(FFM.java:697)
	...

Tested manually to work on Wayland on 1.21, 26.1, 26.2

Related Issue(s)

Checklist

• I made a clear description of what was changed
• I stated why these changes were necessary
• I updated documentation or said what needs to be updated
• I made sure these changes are backwards compatible
• This pull request is for one feature/bug fix