removed MSVC strtok_s use from SDL_strtokr().

no other ??_s are used elsewhere in SDL_stdinc. besides, C11 has a
strtok_s with a different signature.

src/stdlib/SDL_strtokr.c

@@ -28,12 +28,9 @@
 
 char *SDL_strtokr(char *s1, const char *s2, char **ptr)
 {
-#if defined(HAVE_STRTOK_R)
+#ifdef HAVE_STRTOK_R
     return strtok_r(s1, s2, ptr);
 
-#elif defined(_MSC_VER) && defined(HAVE_STRTOK_S)
-    return strtok_s(s1, s2, ptr);
-
 #else /* SDL implementation */
 /*
  * Adapted from _PDCLIB_strtok() of PDClib library at