modem-manager: Fix compiling with old versions of libxmlb
modem-manager: Fix compiling with old versions of libxmlb

Closes: https://github.com/fwupd/fwupd/issues/3600

fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd