darwin: Add fork-inferior.o to NATDEPFILES

darwin: Add fork-inferior.o to NATDEPFILES

I happened to be build-testing on macOS and found this:

  Undefined symbols for architecture x86_64:

    "fork_inferior(char const*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, char**, void (*)(), void (*)(int), void (*)(), char const*, void (*)(char const*, char* const*, char* const*))", referenced from:

        darwin_create_inferior(target_ops*, char const*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, char**, int) in darwin-nat.o

    "startup_inferior(int, int, target_waitstatus*, ptid_t*)", referenced from:

        gdb_startup_inferior(int, int) in fork-child.o

    "trace_start_error(char const*, ...)", referenced from:

        darwin_ptrace_me() in darwin-nat.o

    "trace_start_error_with_name(char const*)", referenced from:

        darwin_ptrace_me() in darwin-nat.o

  ld: symbol(s) not found for architecture x86_64

  clang: error: linker command failed with exit code 1 (use -v to see invocation)

Adding fork-inferior.o fixes it.  I factored out the Darwin bits that
are no architecture-specific in the section meant for that at the top.

I only built-tested this using Travis-CI, since I don't have access to
this platform.

gdb/ChangeLog:

	* configure.nat: Factor out Darwin bits that are not
	architecture-specific.  Add fork-inferior.o.