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/file.cpp

@@ -13,7 +13,7 @@
 #include <sys/types.h>
 #include <unistd.h>
 
-#include "libcamera/internal/file.h"
+#include <libcamera/base/file.h>
 
 #include "test.h"