tcp: Replace 'pragma once' directive with include header guards

The 'pragma once' directive can interact poorly with our build
system in which header files may be found in multiple directories.
The previous commit regarding TCP forward declarations caused some
issues on a macOS machine.

src/internet/model/tcp-prr-recovery.h

@@ -20,7 +20,8 @@
  *         Mohit P. Tahiliani <tahiliani@nitk.edu.in>
  *
  */
-#pragma once
+#ifndef TCP_PRR_RECOVERY_H
+#define TCP_PRR_RECOVERY_H
 
 #include "ns3/tcp-recovery-ops.h"
 
@@ -90,3 +91,4 @@ private:
 };
 } // namespace ns3
 
+#endif /* TCP_PRR_RECOVERY_H */

src/internet/model/tcp-rate-ops.h

@@ -16,7 +16,8 @@
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  *
  */
-#pragma once
+#ifndef TCP_RATE_OPS_H
+#define TCP_RATE_OPS_H
 
 #include "ns3/object.h"
 #include "ns3/tcp-tx-item.h"
@@ -263,3 +264,5 @@ bool operator== (TcpRateLinux::TcpRateSample const & lhs, TcpRateLinux::TcpRateS
 bool operator== (TcpRateLinux::TcpRateConnection const & lhs, TcpRateLinux::TcpRateConnection const & rhs);
 
 } //namespace ns3
+
+#endif /* TCP_RATE_OPS_H */

src/internet/model/tcp-recovery-ops.h

@@ -20,7 +20,8 @@
  *         Mohit P. Tahiliani <tahiliani@nitk.edu.in>
  *
  */
-#pragma once
+#ifndef TCP_RECOVERY_OPS_H
+#define TCP_RECOVERY_OPS_H
 
 #include "ns3/object.h"
 
@@ -193,3 +194,5 @@ public:
 };
 
 } // namespace ns3
+
+#endif /* TCP_RECOVERY_OPS_H */

src/internet/model/tcp-socket-state.h

@@ -15,7 +15,8 @@
  * along with this program; if not, write to the Free Software
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  */
-#pragma once
+#ifndef TCP_SOCKET_STATE_H
+#define TCP_SOCKET_STATE_H
 
 #include "ns3/object.h"
 #include "ns3/data-rate.h"
@@ -243,3 +244,5 @@ namespace TracedValueCallback {
 }  // namespace TracedValueCallback
 
 } //namespace ns3
+
+#endif /* TCP_SOCKET_STATE_H */

src/internet/model/tcp-tx-item.h

@@ -16,7 +16,8 @@
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  *
  */
-#pragma once
+#ifndef TCP_TX_ITEM_H
+#define TCP_TX_ITEM_H
 
 #include "ns3/packet.h"
 #include "ns3/nstime.h"
@@ -113,3 +114,5 @@ private:
 };
 
 } //namespace ns3
+
+#endif /* TCP_TX_ITEM_H */