video/gtk4: Move the dmabuf cfg to the correct bracket level

This was defined one bracket above, which was causing the
gst-gl codepath below to also be disabled when there was
no dmabuf feature enabled.

This was also resulting in the following warning as
we were never creating the MappedFrame::GL vartiant due to this

```
warning: unused variable: `wrapped_context`
   --> video/gtk4/src/sink/frame.rs:541:85
    |
541 | ...", feature = "gst-gl"))] wrapped_context: Option<
    |                             ^^^^^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_wrapped_context`
    |
    = note: `#[warn(unused_variables)]` on by default

warning: variant `GL` is never constructed
  --> video/gtk4/src/sink/frame.rs:80:5
   |
74 | enum MappedFrame {
   |      ----------- variant in this enum
...
```

Move the cfg to the appropriate place where it encaplsulates only
the dmabuf related code.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1682>

video/gtk4/src/sink/frame.rs

@@ -551,10 +551,10 @@ impl Frame {
         #[allow(unused_mut)]
         let mut frame = None;
 
-        #[cfg(all(target_os = "linux", feature = "dmabuf"))]
         {
             // Check we received a buffer with dmabuf memory and if so do some checks before
             // passing it onwards
+            #[cfg(all(target_os = "linux", feature = "dmabuf"))]
             if frame.is_none()
                 && buffer
                     .peek_memory(0)