pdns: Explicitly define lua implementation

pdns: Explicitly define lua implementation

Compile Tested: yes, selects lua as implementation and doesn't pick up luajit
Run Tested: no, minor change

Maintainer: me

Description:
On some architectures PowerDNS was preferring Luajit over Lua, which wasn't added
as a dependency. On previous versions this was controlled by passing `--with-lua`
and `--without-luajit` however this isn't functional anymore.

On the 4.2 series, it is instead possible to define the lua implementation to be
used by passing `--with-lua=[implementation]` ie `--with-lua=lua`

Signed-off-by: James Taylor <redacted>