Style: Fix -Wunused-but-set-variable in test_stream_exceeds_buffer.cpp

This commit addresses the following warnings reported on gcc 5.2.1. In
the future, this will help reduce the "noise" and help catch warnings
revealing a serious problem.

8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---
/path/to/libzmq/tests/test_stream_exceeds_buffer.cpp: In function ‘int main()’:
/path/to/libzmq/tests/test_stream_exceeds_buffer.cpp:19:9: warning: variable ‘rc’ set but not used [-Wunused-but-set-variable]
     int rc = setsockopt(server_sock, SOL_SOCKET, SO_REUSEADDR, &enable, sizeof(enable));
         ^
8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---

tests/test_stream_exceeds_buffer.cpp

@@ -77,5 +77,7 @@ int main()
     assert(rcvbuf[2]==0xbe);
     assert(rcvbuf[1]==0xad);
     assert(rcvbuf[0]==0xde);
+
+    (void)(rc); // avoid -Wunused-but-set-variable warning in release build
 }