Fix compilation without libuv's uv.h header.

ic_proxy_backend.h includes libuv's uv.h header, and ic_proxy_backend.h
was being included in ic_tcp.c, even when compiling with
--disable-ic-proxy.

src/backend/cdb/motion/ic_tcp.c

@@ -28,7 +28,9 @@
 #include "cdb/cdbvars.h"
 #include "cdb/cdbdisp.h"
 
+#ifdef ENABLE_IC_PROXY
 #include "ic_proxy_backend.h"
+#endif  /* ENABLE_IC_PROXY */
 
 #include <fcntl.h>
 #include <limits.h>