motion: fix static build with webp

Static build with webp fails because webp is put before webpmux

Fixes:
 - http://autobuild.buildroot.org/results/4d4e72808300ba1ff79ca794930112b554eb2533



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>