avstring-test: Mark pointer passed to av_free() as non-const

libavutil/avstring.c:278:9: warning: passing argument 1 of ‘av_free’ discards ‘const’ qualifier from pointer target type

libavutil/avstring.c

@@ -270,7 +270,8 @@ int main(void)
 
     printf("Testing av_get_token()\n");
     for (i = 0; i < FF_ARRAY_ELEMS(strings); i++) {
-        const char *p = strings[i], *q;
+        const char *p = strings[i];
+        char *q;
         printf("|%s|", p);
         q = av_get_token(&p, ":");
         printf(" -> |%s|", q);