tftpd_pcre.c: fix build with pcre2 >= 10.43

[ffontaine]

Fix the following build failure raised since pcre2 >= 10.43 and PCRE2Project/pcre2@014c82d:

tftpd_pcre.c: In function 'tftpd_pcre_open':
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:
/home/autobuild/autobuild/instance-10/output-1/host/powerpc64le-buildroot-linux-gnu/sysroot/usr/include/pcre2.h:949:1: note: expected 'PCRE2_UCHAR8 **' {aka 'unsigned char **'} but argument is of type 'const PCRE2_UCHAR8 **' {aka 'const unsigned char **'}
  949 | PCRE2_TYPES_STRUCTURES_AND_FUNCTIONS
      | ^

Fixes:

• http://autobuild.buildroot.org/results/46565c834a8162a651944885104027610a65f9c3