clang: suppress a warning
clang: suppress a warning

Initialize variable explicitly.

    plugin.c:178:44: warning: variable 'plugin' is uninitialized when used here
          [-Wuninitialized]
      if ((id = grn_plugin_find(ctx, filename, plugin))) {
                                               ^~~~~~
    plugin.c:176:22: note: initialize the variable 'plugin' to silence this warning
      grn_plugin **plugin;
                         ^
                          = NULL

cosmo0920/groonga,komainu8/groonga,myokoym/groonga,groonga/groonga,naoa/groonga,naoa/groonga,redfigure/groonga,komainu8/groonga,kenhys/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,naoa/groonga,komainu8/groonga,myokoym/groonga,cosmo0920/groonga,redfigure/groonga,groonga/groonga,cosmo0920/groonga,komainu8/groonga,groonga/groonga,naoa/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,hiroyuki-sato/groonga,redfigure/groonga,myokoym/groonga,groonga/groonga,komainu8/groonga,naoa/groonga,cosmo0920/groonga,cosmo0920/groonga,naoa/groonga,hiroyuki-sato/groonga,kenhys/groonga,myokoym/groonga,hiroyuki-sato/groonga,hiroyuki-sato/groonga,komainu8/groonga,komainu8/groonga,naoa/groonga,redfigure/groonga,groonga/groonga,redfigure/groonga,hiroyuki-sato/groonga,groonga/groonga,komainu8/groonga,groonga/groonga,redfigure/groonga,myokoym/groonga,kenhys/groonga,cosmo0920/groonga,kenhys/groonga,myokoym/groonga,myokoym/groonga,myokoym/groonga,redfigure/groonga,cosmo0920/groonga,naoa/groonga,redfigure/groonga,kenhys/groonga,kenhys/groonga,hiroyuki-sato/groonga