build: fix imgui on GTK

src/apprt/gtk/class/imgui_widget.zig

@@ -1,7 +1,7 @@
 const std = @import("std");
 const assert = @import("../../../quirks.zig").inlineAssert;
 
-const cimgui = @import("cimgui");
+const cimgui = @import("dcimgui");
 const gl = @import("opengl");
 const adw = @import("adw");
 const gdk = @import("gdk");

src/build/SharedDeps.zig

@@ -484,6 +484,7 @@ pub fn add(
         .optimize = optimize,
         .@"backend-metal" = target.result.os.tag.isDarwin(),
         .@"backend-osx" = target.result.os.tag == .macos,
+        .@"backend-opengl3" = target.result.os.tag != .macos,
     })) |dep| {
         step.root_module.addImport("dcimgui", dep.module("dcimgui"));
         step.linkLibrary(dep.artifact("dcimgui"));