cups: broken Zebra support without cups-dev

Without installing cups-dev, there's no Zebra choice in the driver picker. This is caused by Alpine packaging /usr/share/cups/ppdc/label.h with cups-dev, along with hp.h and epson.h. It seems that CUPS uses those files at runtime to compile PPD drivers. The fix would be to move those files to cups-filters or main cups package.

cc: @psykose (also thanks for helping me debug this ^^)