It might be better to use piCore-11.x - I tested the rpi3 built-in wifi a week ago and it worked fine.
If you still wish to use piCore-10.x, the wireless drivers are available for linux-4.19.58, so you probably need to update your version of piCore-10.x.
Otherwise, you need to symlink /lib/modules/4.19.46-piCore-v7/build to your kernel source, for example: $ sudo ln -s /usr/src/linux-4.19.46 /lib/modules/4.19.46-piCore-v7/build