Docs: Use kernel-figure in vidioc-g-selection.rst

...otherwise the PDF build fails when it can't find constraints.pdf.
Acked-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/media/uapi/v4l/vidioc-g-selection.rst

@@ -129,8 +129,8 @@ Selection targets and flags are documented in
 
 .. _sel-const-adjust:
 
-.. figure::  constraints.*
-    :alt:    constraints.pdf / constraints.svg
+.. kernel-figure::  constraints.svg
+    :alt:    constraints.svg
     :align:  center
 
     Size adjustments with constraint flags.