Add missing 'static' to BPP vtable definitions.

Vtable objects only need to be globally visible throughout the code if
they're used directly in some interchangeable way, e.g. by passing
them to a constructor like cipher_new that's the same for all
implementations of the vtable, or by directly looking up public data
fields in the vtable itself.

But the BPPs are never used like that: each BPP has its own
constructor function with a different type signature, so the BPP types
are not interchangeable in any way _before_ an instance of one has
been constructed. Hence, their vtable objects don't need external
linkage.

sshverstring.c

@@ -44,7 +44,7 @@ static void ssh_verstring_handle_input(BinaryPacketProtocol *bpp);
 static PktOut *ssh_verstring_new_pktout(int type);
 static void ssh_verstring_format_packet(BinaryPacketProtocol *bpp, PktOut *);
 
-const struct BinaryPacketProtocolVtable ssh_verstring_vtable = {
+static const struct BinaryPacketProtocolVtable ssh_verstring_vtable = {
     ssh_verstring_free,
     ssh_verstring_handle_input,
     ssh_verstring_new_pktout,