libzmqutil: use size_t not int

Problem: a function in the internal mpart class uses an int type where
size_t would be more appropriate.

Switch to size_t.

src/common/libzmqutil/mpart.c

@@ -102,7 +102,7 @@ static int mpart_append (zlist_t *mpart, zmq_msg_t *part)
     return 0;
 }
 
-int mpart_addmem (zlist_t *mpart, const void *buf, int size)
+int mpart_addmem (zlist_t *mpart, const void *buf, size_t size)
 {
     zmq_msg_t *part;
 

src/common/libzmqutil/mpart.h

@@ -20,7 +20,7 @@
  */
 void mpart_destroy (zlist_t *mpart);
 zlist_t *mpart_create (void);
-int mpart_addmem (zlist_t *mpart, const void *buf, int size);
+int mpart_addmem (zlist_t *mpart, const void *buf, size_t size);
 int mpart_addstr (zlist_t *mpart, const char *s);
 zlist_t *mpart_recv (void *sock);
 int mpart_send (void *sock, zlist_t *mpart);