This way the object file correctly depends on external headers. Currently
a parallel build on a clean tree can give:
```
In file included from ./common/sphinx.h:6:0,
from devtools/onion.c:5:
./bitcoin/pubkey.h:8:10: fatal error: secp256k1.h: No such file or directory
#include <secp256k1.h>
^~~~~~~~~~~~~
compilation terminated.
<builtin>: recipe for target 'devtools/onion.o' failed
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
These were so far only used for bolt11 construction, but we'll need them for the
DNS seed as well, so here we just pull them out into their own unit and prefix
them.
Signed-off-by: Christian Decker <decker.christian@gmail.com>
