tgetopt: use const qualifier from pointer target type

[uraltun]

Fixes a build failure against glibc 2.43:

/<>/programs/tgetopt.c: In function ‘tgetopt’: /<>/programs/tgetopt.c:73:36: error: initialization discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
73 | tchar *p = tstrchr(optstring, opt);
| ^~~~~~~

[uraltun]

Log of successful build including this change against glibc 2.43: https://launchpadlibrarian.net/849909508/buildlog_ubuntu-resolute-amd64.libdeflate_1.23-2ubuntu1~ppa1_BUILDING.txt.gz

[ebiggers]

Merged, thanks. For future reference, there is more information in this glibc commit:

commit cd748a63ab1a7ae846175c532a3daab341c62690
Author: Joseph Myers <josmyers@redhat.com>
Date:   Thu Nov 20 19:30:27 2025 +0000

    Implement C23 const-preserving standard library macros
    
    C23 makes various standard library functions, that return a pointer
    into an input array, into macros that return a pointer to const when
    the relevant argument passed to the macro is a pointer to const.