darwin-nat: Add missing include

darwin-nat: Add missing include

I forgot this one, which is kind of related.

The function trace_start_error_with_name has moved in commit "Share
fork_inferior et al with gdbserver", so this additional include is
needed.

Fixes:

darwin-nat.c:1735:5: error: use of undeclared identifier 'trace_start_error_with_name'
    trace_start_error_with_name ("close");

gdb/ChangeLog:

	* darwin-nat.c: Include "nat/fork-inferior.h".