gio: Move conditionally used imports to the place where they're used

Otherwise they're unused when compiling without feature flags enabled.

Author: sdroege

gio/tests/std_io_copy.rs

@@ -1,10 +1,9 @@
-use std::io;
-
-use gio::prelude::*;
-
 #[test]
 #[cfg(feature = "v2_36")]
 fn std_io_copy_with_gio() {
+    use gio::prelude::*;
+    use std::io;
+
     let bytes = glib::Bytes::from_owned([1, 2, 3]);
     let mut read = gio::MemoryInputStream::new_from_bytes(&bytes).into_read();
     let mut write = gio::MemoryOutputStream::new_resizable().into_write();