pipeline: rpi: pisp: Fix uninitialized variable warning

gcc 13.3.0 from buildroot 2024.11.1 complains about an uninitialized
variable. This is a false positive as the cfe_ array can't be empty.
Nonetheless, it breaks builds, so initialize the variable to work around
the issue.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/pipeline/rpi/pisp/pisp.cpp

@@ -1700,7 +1700,7 @@ void PiSPCameraData::platformFreeBuffers()
 void PiSPCameraData::cfeBufferDequeue(FrameBuffer *buffer)
 {
 	RPi::Stream *stream = nullptr;
-	int index;
+	int index = 0;
 
 	if (!isRunning())
 		return;