package/imagemagick: add optional support for lcms2

When lcms2 was compiled before, imagemagick will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/identify | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libMagickCore-6.Q16.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libMagickWand-6.Q16.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [liblcms2.so.2]
[...]

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