build: Fix PNG image generation
build: Fix PNG image generation

The 'dest' option is apparently relative to repository root and gets
appended with the full path from the 'src' option for each file.

Change-Id: If6d4e0565d222b90f0b666a749c61f950575d451

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui