Fix broken zig 0.14 build due to bad zsdl dependency

[mjakeman]

Fixes an issue where zgui brings in a non-0.14 compatible version of zsdl, breaking the final build.

This only seems to happen if you use all of zgui + zsdl + zsdl-prebuilt*.

My understanding of the problem is:

• zsdl was updated to support the new build.zig syntax in zig-gamedev/zsdl@b080476
• However zgui has an optional dependency pointing to zig-gamedev/zsdl@cd29a0b which doesn't include these changes
• Zig decides to pull in both versions of zsdl, therefore pulling in the old and new versions of the prebuilt libraries at the same time.
• The old prebuilt libraries use .@"sdl2-prebuilt-macos" which isn't supported, but Zig appears to suggest the error is coming from the new library (or at least is confusingly worded)

Bumping the dependency fixes things for me