Build: Do not build libdav1dentrypoint by default

This library is just a helper and should not be built by default as it
is only used to extract the objects and has no use as stand-alone
library for uses to link to.
3 jobs for itx_vla in 38 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #538
debian amd64
build-debian

00:00:36

passed #539
win32
build-win32

00:00:35

passed #540
win64
build-win64

00:00:38