sys/cbor: change array initializer from const variable to #define

OS X clang complains "folded to constant array as an extension"
[-Wgnu-folding-constant]

sys/cbor/cbor.c

@@ -81,6 +81,9 @@
 /* Extra defines not related to the protocol itself */
 #define CBOR_STREAM_PRINT_BUFFERSIZE 1024 /* bytes */
 
+/* Array size */
+#define MAX_TIMESTRING_LENGTH   (21)
+
 #ifndef INFINITY
 #define INFINITY (1.0/0.0)
 #endif
@@ -694,7 +697,6 @@ size_t cbor_deserialize_date_time(const cbor_stream_t *stream, size_t offset, st
 
 size_t cbor_serialize_date_time(cbor_stream_t *stream, struct tm *val)
 {
-    static const int MAX_TIMESTRING_LENGTH = 21;
     CBOR_ENSURE_SIZE(stream, MAX_TIMESTRING_LENGTH + 1); /* + 1 tag byte */
 
     char time_str[MAX_TIMESTRING_LENGTH];