package/leptonica: add libwebp{demux, mux} optional dependency

leptonica optionally depends on webp/demux.h and webp/mux.h since
version 1.79.0:
https://github.com/DanBloomberg/leptonica/commit/a7c5bcdf04062aad3655642bdb853561a1c49733
https://github.com/DanBloomberg/leptonica/commit/0ce4b9cc0845b198583d51f0a0df277de8f8c5fa

Fixes:
 - http://autobuild.buildroot.org/results/1f8531e1651e82a1e93707ccb205d0e7a3cae1a3



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