Define BITTORRENT_NULL_ID in common.c; declare extern in common.h.
This change avoids linker warnings when building with Debian tcc
0.9.23-4 + patch from Debian bug 418360:
[LD] src/protocol/bittorrent/lib.o
bittorrent.o: 'BITTORRENT_NULL_ID' defined twice
common.o: 'BITTORRENT_NULL_ID' defined twice
connection.o: 'BITTORRENT_NULL_ID' defined twice
dialogs.o: 'BITTORRENT_NULL_ID' defined twice
peerconnect.o: 'BITTORRENT_NULL_ID' defined twice
peerwire.o: 'BITTORRENT_NULL_ID' defined twice
piececache.o: 'BITTORRENT_NULL_ID' defined twice
tracker.o: 'BITTORRENT_NULL_ID' defined twice