journalctl: make libqrencode a weak dependency

journalctl: make libqrencode a weak dependency

This way journalctl can make use of libqrencode if it's there, but will
quietly not use it if it isn't.

This means libqrencode remains a build-time dep, but not a strict
runtime dependency.

I figure we should do something similar for a bunch of other "leaf"
libraries we only use few symbols of. Specifically the following are
probably good candidates:

* pcre2
* libpwquality
* p11kit
* elfutils

and possibly:

* libcryptsetup (only in some parts. i.e. building systemd-cryptsetup
  without it makes no sense. However building the dissect option with
  libcryptsetup as optional dep does make sense)
* possibly the compression libraries (at least the ones we never use for
  compression, but only as alternative ones for decompression)

Already covered like this is:

* libxkcommon