fonts: fix BuilderBlocks not loading on Win32 renderer

BuilderBlocks failed to load when using the native Win32 font renderer, due to missing family name.

Fixes: gnome-text-editor#585 (closed)

Tested on:

• Linux (fc renderer)
• Windows10 (native win32 renderer)
• Windows10 (fc renderer)

Anyone uses gtksourceview on Mac? would be nice to give a try before merging, thanks!