Avoid multiple definitions of `struct tcphdr`.

No-one's hit this in practice, that I know of, but there are very few
instances of this old workaround for kernel/userspace mismatches still
present, and (as part of the much harder and less effective `struct
sigaction` cleanup), we should just remove them.

Bug: http://b/236042740
Test: treehugger
Change-Id: I6c71d4353044cf57cfa8a9796a4c3d6a4d51cd86
diff --git a/libc/kernel/tools/defaults.py b/libc/kernel/tools/defaults.py
index ac12fc7..e2426c6 100644
--- a/libc/kernel/tools/defaults.py
+++ b/libc/kernel/tools/defaults.py
@@ -43,6 +43,7 @@
     "in_addr": False,
     "ip_mreq_source": False,
     "ip_msfilter": False,
+    "tcphdr": False,
     "timespec": False,
     }