Fix strpbrk() discarded const qualifier

(cherry picked from commit c45be94938)

src/stdlib/SDL_string.c

@@ -2479,7 +2479,7 @@ int SDL_vasprintf(char **strp, SDL_PRINTF_FORMAT_STRING const char *fmt, va_list
 char * SDL_strpbrk(const char *str, const char *breakset)
 {
 #ifdef HAVE_STRPBRK
-    return strpbrk(str, breakset);
+    return SDL_const_cast(char *, strpbrk(str, breakset));
 #else
 
     for (; *str; str++) {