Meta/gn: Move enable_qt arg into dedicated gni file

No behavior change.

Meta/gn/secondary/Ladybird/BUILD.gn

@@ -1,12 +1,8 @@
 import("//Ladybird/compile_qt_resource_file.gni")
+import("//Ladybird/enable_qt.gni")
 import("//Ladybird/link_qt.gni")
 import("//Ladybird/moc_qt_objects.gni")
 
-declare_args() {
-  # Build the Ladybird application using the Qt chrome.
-  enable_qt = current_os != "mac"
-}
-
 group("Ladybird") {
   if (current_os == "mac") {
     deps = [ ":Ladybird.app" ]

Meta/gn/secondary/Ladybird/enable_qt.gni

@@ -0,0 +1,4 @@
+declare_args() {
+  # Build the Ladybird application using the Qt chrome.
+  enable_qt = current_os != "mac"
+}