package/efl/libefl: add webp dependency

WebP is a new image format that provides lossless and lossy
compression for images on the web. So enabling webp support in efl
libraries allow to loads images using WebP.

Also, it one of the "highly recommended" dependencies [1] according to
the README but disabling it doesn't need the
--enable-i-really-know-what-i-am-doing... option.  That's why
BR2_PACKAGE_LIBEFL_WEBP is not added to
BR2_PACKAGE_LIBEFL_HAS_RECOMMENDED_CONFIG.

[1] https://git.enlightenment.org/core/efl.git/tree/README?id=v1.15.2#n486



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>