Don't use "size_t" for the nbytes field in the header - use uint32_t to ensure that ntohl/htonl correctly match it

Refs trac:4026

This commit was SVN r30062.

The following Trac tickets were found above:
  Ticket 4026 --> https://svn.open-mpi.org/trac/ompi/ticket/4026

orte/mca/oob/tcp/oob_tcp_hdr.h

@@ -55,7 +55,7 @@ typedef struct {
     /* the rml tag where this message is headed */
     orte_rml_tag_t tag;
     /* number of bytes in message */
-    size_t nbytes;
+    uint32_t nbytes;
 } mca_oob_tcp_hdr_t;
 /**
  * Convert the message header to host byte order