only expose struct tcphdr under _GNU_SOURCE

the BSD and GNU versions of this structure differ, so exposing it in
the default _BSD_SOURCE profile is possibly problematic. both versions
could be simultaneously supported with anonymous unions if needed in
the future, but for now, just omitting it except under _GNU_SOURCE
should be safe.

include/netinet/tcp.h

@@ -31,8 +31,10 @@
 #define SOL_TCP 6
 #include <sys/types.h>
 #include <sys/socket.h>
-#include <endian.h>
+#endif
 
+#ifdef _GNU_SOURCE
+#include <endian.h>
 struct tcphdr
 {
 	u_int16_t source;