meson: use jinja2 also for custom-entities.ent

meson: use jinja2 also for custom-entities.ent

This doesn't matter too much, but makes things a bit more consistent.
A minor advantage is that the file is not a configuration file for meson
anymore, so:
 a) It is not built unless pulled in by another target. Since
    we don't usually build man pages by default, this saves a tiny
    amount of work.
 b) When the .in file is updated, meson does not reconfigure everything,
    but just rebuilds the dependent targets.

Now that the conversion is finished, time for benchmarking:
a full build with default settings (and -Dstandalonebinaries=true), yields

before this pull request: 1687 targets, 148.13s user 35.17s system 317% cpu 57.697 total
with the full pull request: 1714 targets, 143.07s user 27.87s system 314% cpu 54.369 total

The difference doesn't seem significant. Partial rebuilds might be faster as
mentioned before.