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

lib/Makefile

···
 #
 obj-$(CONFIG_OID_REGISTRY) += oid_registry.o
 
-$(obj)/oid_registry.c: $(obj)/oid_registry_data.c
+$(obj)/oid_registry.o: $(obj)/oid_registry_data.c
 
 $(obj)/oid_registry_data.c: $(srctree)/include/linux/oid_registry.h \
 			    $(src)/build_OID_registry