Fix compilation warnings with unique_command_from_string

xs/UniqueApp.xs: In function ‘XS_Gtk2__UniqueApp_send_message_by_name’:
xs/UniqueApp.xs:323:19: warning: implicit declaration of function ‘unique_command_from_string’ [-Wimplicit-function-declaration]
      command_id = unique_command_from_string(app, command_name);