meson: fix perl detection when docs are enabled, but perl bindings aren't

meson: fix perl detection when docs are enabled, but perl bindings aren't

The `perl` variable in meson.build is assigned to a program lookup,
which may have the value "not-found object" if configuring with
`-Dperl=disabled`.

There is already a list of other cases where we do need a perl command,
even when not building perl bindings. Building documentation should be
one of those cases, but was missing from the list. Add it.

Fixes:

```
$ meson setup builddir/ -Ddocs=man -Dperl=disabled -Dtests=false
[...]
Documentation/meson.build:308:22: ERROR: Tried to use not-found external program in "command"
```

Bug: https://bugs.gentoo.org/949247
Signed-off-by: Eli Schwartz <eschwartz@gentoo.org>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>