gssapi: add zmq_gssapi.7 to MAN7 in Makefile.am

Problem: zmq_gssapi.7 was not mentioned in doc/Makefile.am

Solution: add man page to MAN7 in doc/Makefile.am

doc/Makefile.am

@@ -21,7 +21,8 @@ MAN3 = zmq_bind.3 zmq_unbind.3 zmq_connect.3 zmq_disconnect.3 zmq_close.3 \
     zmq_atomic_counter_value.3 zmq_atomic_counter_destroy.3
 
 MAN7 = zmq.7 zmq_tcp.7 zmq_pgm.7 zmq_inproc.7 zmq_ipc.7 \
-    zmq_null.7 zmq_plain.7 zmq_curve.7 zmq_tipc.7 zmq_vmci.7 zmq_udp.7
+    zmq_null.7 zmq_plain.7 zmq_curve.7 zmq_tipc.7 zmq_vmci.7 zmq_udp.7 \
+    zmq_gssapi.7
 
 MAN_DOC =