Fix warnings with gcc 15 and glibc 2.43.

Fix warnings with gcc 15 and glibc 2.43.

* libtextstyle/gnulib-local/lib/glib/gstrfuncs.in.h (g_strstr_len): Change
return type to 'const gchar *'.
* libtextstyle/gnulib-local/lib/glib/gstrfuncs.c (g_strstr_len): Likewise.
* libtextstyle/gnulib-local/lib/libcroco/cr-rgb.c (cr_rgb_set_from_name): Change
type of 'result'.
* gettext-tools/src/cldr-plurals.c (extract_rules): Cast the sb_xcontents_c
result to 'char *'.