Make cmake stop complaining about freetype not being exported

[jonesmz]

A bit of a hack, but it fixes this cmake error for my local build.

[jonesmz]

Note that this only works with a version of freetype that's either imported from the filesystem, or if it's in the same build tree, it needs to be built with BUILD_SHARED_LIBS = off. Otherwise you still need to export the shared freetype library.

[mikke89]

Note that this only works with a version of freetype that's either imported from the filesystem, or if it's in the same build tree, it needs to be built with BUILD_SHARED_LIBS = off. Otherwise you still need to export the shared freetype library.

Hm, I think CMake does the right thing when linking libraries, so having FreeType as PRIVATE is correct in this case? FreeType is not part of the public interface of RmlUi, so to me I think this one is good to merge.