Update copyright years in libobjc.

libobjc/ChangeLog

libobjc/Makefile.in

libobjc/NXConstStr.m

libobjc/Object.m

libobjc/Protocol.m

libobjc/accessors.m

libobjc/acinclude.m4

libobjc/class.c

libobjc/configure.ac

libobjc/encoding.c

libobjc/error.c

libobjc/exception.c

libobjc/gc.c

libobjc/hash.c

libobjc/init.c

libobjc/ivars.c

libobjc/libobjc.def

libobjc/linking.m

libobjc/memory.c

libobjc/methods.c