vwm: fix build against ncurses-6.3
vwm: fix build against ncurses-6.3

Without the change the build on upcoming `ncurses-6.3` fails as:

    vwm_bkgd.c:71:63: error: format not a string literal and no format arguments [-Werror=format-security]
       71 |         mvwprintw(window,height-1,width-(strlen(version_str)),version_str);
          |                                                               ^~~~~~~~~~~

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