UTF8_CHK_SKIP: eliminate casting away const

Seen with:

  ./Configure -des -Dusedevel -Accflags=-Wcast-qual

mathoms.c: In function ‘Perl_utf8_to_uvchr’:
utf8.h:804:53: warning: cast discards ‘const’ qualifier from pointer target type
 [-Wcast-qual]
  804 |            (UNLIKELY(s[0] == '\0') ? 1 : my_strnlen((char *) (s), UTF8SK
IP(s)))
      |                                                     ^
embed.h:792:87: note: in definition of macro ‘utf8_to_uvchr_buf_helper’
  792 | # define utf8_to_uvchr_buf_helper(a,b,c)        Perl_utf8_to_uvchr_buf_h
elper(aTHX_ a,b,c)
      |
              ^
mathoms.c:970:12: note: in expansion of macro ‘utf8_to_uvchr_buf’
  970 |     return utf8_to_uvchr_buf(s, s + UTF8_CHK_SKIP(s), retlen);
      |            ^~~~~~~~~~~~~~~~~
mathoms.c:970:37: note: in expansion of macro ‘UTF8_CHK_SKIP’
  970 |     return utf8_to_uvchr_buf(s, s + UTF8_CHK_SKIP(s), retlen);
      |                                     ^~~~~~~~~~~~~

mathoms.c seems to be the only user of this macro.

This came up when reviewing Perl#22271

utf8.h

@@ -801,7 +801,7 @@ C<L</UTF8_SAFE_SKIP>>, for example when interfacing with a C library.
 */
 
 #define UTF8_CHK_SKIP(s)                                                       \
-           (UNLIKELY(s[0] == '\0') ? 1 : my_strnlen((char *) (s), UTF8SKIP(s)))
+           (UNLIKELY(s[0] == '\0') ? 1 : my_strnlen((const char *) (s), UTF8SKIP(s)))
 /*
 
 =for apidoc Am|STRLEN|UTF8_SAFE_SKIP|char* s|char* e