xenial: fix clang error messages

* Remove FUNC_ATTR_NONNULL_ALL from function without pointer arguments.

  Otherwise the ASAN build would complain:

  error: 'nonnull' attribute applied to function with no pointer arguments [-Werror,-Wignored-attributes]
  static void do_autocmd_focusgained(_Bool gained) __attribute__((nonnull));

src/nvim/aucmd.c

@@ -26,7 +26,6 @@ void aucmd_schedule_focusgained(bool gained)
 }
 
 static void do_autocmd_focusgained(bool gained)
-  FUNC_ATTR_NONNULL_ALL
 {
   static bool recursive = false;