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