linux_dep: Generalise tcp_info.h to handling Linux extension compatibility

tcp_info.h exists just to contain a modern enough version of struct
tcp_info for our needs, removing compile time dependency on the version of
kernel headers.  There are several other cases where we can remove similar
compile time dependencies on kernel version.  Prepare for that by renaming
tcp_info.h to linux_dep.h.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

tcp.c

@@ -299,10 +299,10 @@
 #include "log.h"
 #include "inany.h"
 #include "flow.h"
+#include "linux_dep.h"
 
 #include "flow_table.h"
 #include "tcp_internal.h"
-#include "tcp_info.h"
 #include "tcp_buf.h"
 
 /* MSS rounding: see SET_MSS() */