Fix implicit definition of strverscmp.

[Ferada]

Gets rid of this warning:

filelist.c: In function ‘strcmp_or_strverscmp’:
filelist.c:414:10: warning: implicit declaration of function ‘strverscmp’; did you mean ‘strncmp’? [-Wimplicit-function-declaration]
  414 |   return(strverscmp(s1, s2));
      |          ^~~~~~~~~~
      |          strncmp

[derf]

Thanks!