locale.c: Fix compilation error on some platforms

Commit 73d66f3 introduced a compilation
error into some platforms lacking nl_langinfo(3)  (generally Windows,
but their compiler is more lenient in this regard.

Simply add a 'const' to the declaration to fix it.

Author: khwilliamson

locale.c

@@ -4467,7 +4467,7 @@ S_my_langinfo_i(pTHX_
         retval++;
 
         /* And stop before any '@' */
-        char * modifier = strchr(retval, '@');
+        const char * modifier = strchr(retval, '@');
         if (modifier) {
             char * code_set_name;
             const Size_t name_len = modifier - retval;