Fix FTBFS after 9dfa154413356e4247ff2d99adc2557c58711cbc
Fix FTBFS after 9dfa154413356e4247ff2d99adc2557c58711cbc

The commit in issue replaces the second parameter from 0 to nullptr. As
the second parameter `mode` is actually an int variable, the build
fails:

/build/lxqt-panel-git/src/lxqt-panel/plugin-volume/alsaengine.cpp:169:43: error: cannot convert ‘std::nullptr_t’ to ‘int’
             snd_mixer_open(&mixer, nullptr);
                                           ^

Reverted the problematic line to make lxqt-panel build again.

Cc: 5acb2a96dfb3a907efe6ece42a99c669a23bbc23@tsujan
Ref: https://www.alsa-project.org/alsa-doc/alsa-lib/group___mixer.html#ga240cfebce7d68f22875d76e99cf19394

stefonarch/lxqt-panel,stefonarch/lxqt-panel,stefonarch/lxqt-panel