Move libsqlite3 to the top of the SUBDIR list

In parallel builds, this should allow sqlite to start building earlier and
increase parallelism when building lib/. Looking at htop output during
buildworld/tinderbox, there are long phases where only one CPU is active
optimizing the massive sqlite3.c file since the build of libsqlite3 is
started quite late.

Reviewed By: emaste
Differential Revision: https://reviews.freebsd.org/D26169