systemc: Change the type of a loop counter to avoid a warning.

g++ complained about comparing an signed int loop counter with the
return value of a size() function. This change changes it to an
unsigned to make g++ happy/quiet.

Change-Id: I28fa79c448465b24d77b5623860f9b991f313561
Reviewed-on: https://gem5-review.googlesource.com/c/16286
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/systemc/ext/core/sc_port.hh

@@ -230,7 +230,7 @@ class sc_port_b : public sc_port_base
     {
         IF *interface = dynamic_cast<IF *>(iface);
         sc_assert(interface);
-        for (int i = 0; i < _interfaces.size(); i++) {
+        for (unsigned i = 0; i < _interfaces.size(); i++) {
             if (interface == _interfaces[i]) {
                 report_error(SC_ID_BIND_IF_TO_PORT_,
                         "interface already bound to port");