package/imagemagick: add optional support for webp

When webp was compiled before, imagemagick will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/stream | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libMagickCore-6.Q16.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libMagickWand-6.Q16.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libjpeg.so.8]
 0x0000000000000001 (NEEDED)             Shared library: [libwebp.so.5]
[...]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>