Fix compilation on platforms using gcc.

When compiling RACK on platforms using gcc, a warning that tcp_outflags
is defined but not used is issued and terminates compilation on PPC64,
for example. So don't indicate that tcp_outflags is used.

Reviewed by:		rrs@
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D20971

sys/netinet/tcp_stacks/rack_bbr_common.c

@@ -85,7 +85,6 @@ __FBSDID("$FreeBSD$");
 #include <netinet/ip6.h>
 #include <netinet6/in6_pcb.h>
 #include <netinet6/ip6_var.h>
-#define	TCPOUTFLAGS
 #include <netinet/tcp.h>
 #include <netinet/tcp_fsm.h>
 #include <netinet/tcp_seq.h>