Fix GN undefined identifier error on non-Linux platforms

BUILD.gn

@@ -235,6 +235,7 @@ if (is_win) {
   import("//build/config/win/manifest.gni")
 }
 
+if (is_linux) {
   declare_args() {
     # The cefclient target depends on GTK packages that are not available in the
     # default sysroot environment. So we should only use them when we are not
@@ -242,6 +243,7 @@ declare_args() {
     # GTK dependencies at all, in which case they can set `cef_use_gtk=false`.
     cef_use_gtk = !use_sysroot
   }
+}
 
 #
 # Verify required global arguments configured via `gn args`.