lib/Makefile: Fix oid_registry build dependency

lib/Makefile: Fix oid_registry build dependency

It is $(obj)/oid_registry.o that is dependent on $(obj)/oid_registry_data.c.
The object file cannot be built until $(obj)/oid_registry_data.c has been
generated.

A periodic and hard to reproduce parallel build failure is due to
this incorrect lib/Makefile dependency. The compile error is completely
disingenuous.

  GEN     lib/oid_registry_data.c
Compiling 49 OIDs
  CC      lib/oid_registry.o
gcc: error: lib/oid_registry.c: No such file or directory
gcc: fatal error: no input files
compilation terminated.
make[3]: *** [lib/oid_registry.o] Error 4

Cc: Andrew Morton <[email protected]>
Cc: Akinobu Mita <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: David Howells <[email protected]>
Cc: "David S. Miller" <[email protected]>
Signed-off-by: Tim Gardner <[email protected]>
Signed-off-by: Rusty Russell <[email protected]>