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 master in 37 seconds
Status Name Job ID Coverage
  Build
passed build-debian #532
debian amd64

00:00:37

passed build-win32 #533
win32

00:00:35

passed build-win64 #534
win64

00:00:36