libcamera/base: Move File to base library

The File abstraction is a base helper and not part of the libcamera
API. Move it to to allow usage by users of the base library.

Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Paul Elder  <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

test/hotplug-cameras.cpp

@@ -15,11 +15,10 @@
 #include <libcamera/camera_manager.h>
 
 #include <libcamera/base/event_dispatcher.h>
+#include <libcamera/base/file.h>
 #include <libcamera/base/thread.h>
 #include <libcamera/base/timer.h>
 
-#include "libcamera/internal/file.h"
-
 #include "test.h"
 
 using namespace libcamera;