Cleanup: don't use GNU old-style field designator extension

Cleanup: don't use GNU old-style field designator extension

ustfork.c:112:38: warning: use of GNU old-style field designator extension
      [-Wgnu-designator]
                struct ustfork_clone_info info = { fn: fn, arg: arg };
                                                   ^~~
                                                   .fn =
ustfork.c:112:46: warning: use of GNU old-style field designator extension
      [-Wgnu-designator]
                struct ustfork_clone_info info = { fn: fn, arg: arg };
                                                           ^~~~
                                                           .arg =

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>