test: gstreamer: Drop internal header from base class

The internal header isn't needed. The needed function
libcameraBuildPath() is exposed by libcamera/base/utils.h header.
At the same time, move the utils header to .cpp instead of including
it in the base class header itself.

Signed-off-by: Umang Jain <umang.jain@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

test/gstreamer/gstreamer_test.cpp

@@ -5,6 +5,8 @@
  * libcamera Gstreamer element API tests
  */
 
+#include <libcamera/base/utils.h>
+
 #include "gstreamer_test.h"
 
 #include "test.h"