Add missing extern qualifiers for gcc 10

GCC 10 defaults to `-fno-common`. no longer automatically sharing
global variable definitions, which makes it important to define
them in only one place (otherwise there will be duplicate definition
errors). Add `extern` qualifiers where (I think) is the best place for
them.

common/daemon.h

@@ -20,6 +20,4 @@ void daemon_shutdown(void);
 /* Kick in a debugger if they set --debugger */
 void daemon_maybe_debug(char *argv[]);
 
-struct backtrace_state *backtrace_state;
-
 #endif /* LIGHTNING_COMMON_DAEMON_H */