util: added strtok_r implementation for old compilers

Visual studios older than 2005 don't have strtok_s() and according to
MSDN vs2005 has NOT thread-safe strtok().

src/util.h

@@ -18,6 +18,9 @@
 
 #include "ostypes.h"
 
+/* string functions */
+char *xmpp_strtok_r(char *s, const char *delim, char **saveptr);
+
 /* timing functions */
 uint64_t time_stamp(void);
 uint64_t time_elapsed(uint64_t t1, uint64_t t2);