Add missing header guard to new header files

lib/isc/include/isc/astack.h

@@ -9,6 +9,8 @@
  * information regarding copyright ownership.
  */
 
+#pragma once
+
 #include <inttypes.h>
 #include <isc/mem.h>
 #include <isc/types.h>

lib/isc/include/isc/hp.h

@@ -43,6 +43,8 @@
  * THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#pragma once
+
 #include <isc/atomic.h>
 #include <isc/string.h>
 #include <isc/mem.h>

lib/isc/netmgr/netmgr-int.h

@@ -9,6 +9,8 @@
  * information regarding copyright ownership.
  */
 
+#pragma once
+
 #include <unistd.h>
 #include <uv.h>