Skip extra fonts if not an extended font build.

This fixes build warning:
build/core/sdk_font.mk:51: warning: overriding commands for target
`out/host/common/obj/PACKAGING/sdk-fonts_intermediates/NanumGothic.ttf'

Change-Id: I5e208faf25f6f5d1ae3fac9970c70e80e42c7dcd