Fix incompatible-pointer-types for pcre2_substring_list_free

This change fixes the following compilation build error for gcc14:

tftpd_pcre.c:109:37: error: passing argument 1 of 'pcre2_substring_list_free_8' from incompatible pointer type [-Wincompatible-pointer-types]
  109 |           pcre2_substring_list_free((const PCRE2_UCHAR **)substrlist);
      |                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                     |
      |                                     const PCRE2_UCHAR8 ** {aka const unsigned char **}
In file included from tftpd_pcre.h:24,
                 from tftpd_pcre.c:35:

Signed-off-by: Florian Eckert <[email protected]>

Author: feckert

tftpd_pcre.c

@@ -106,7 +106,7 @@ tftpd_pcre_self_t *tftpd_pcre_open(char *filename)
                logger(LOG_DEBUG,"file: %s line: %d substring: %d value: %s",
                       filename, linecount, subnum, substrlist[subnum]);
           }
-          pcre2_substring_list_free((const PCRE2_UCHAR **)substrlist);
+          pcre2_substring_list_free((PCRE2_UCHAR **)substrlist);
 
           if (matches != 3)
           {