test: Ensure LIBCAMERA_BASE_PRIVATE isn't public

If LIBCAMERA_BASE_PRIVATE is ever exposed on the libcamera public dependencies,
then the private.h header protection will be circumvented.

Provide a test which will fail (at compile time) if the LIBCAMERA_BASE_PRIVATE
define ever leaks to the public dependencies.

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/meson.build

@@ -25,6 +25,7 @@ subdir('v4l2_videodevice')
 
 public_tests = [
     ['geometry',                        'geometry.cpp'],
+    ['public-api',                      'public-api.cpp'],
     ['signal',                          'signal.cpp'],
     ['span',                            'span.cpp'],
 ]