gdb: For macOS, s/thread_info/struct thread_info/

The macOS build currently fails with several instances of this problem:

  In file included from ../../src/gdb/darwin-nat.h:22:0,
		   from ../../src/gdb/i386-darwin-nat.c:37:
  ../../src/gdb/gdbthread.h:376:59: error: type/value mismatch at argument 1 in template parameter list for 'template<class T, class Policy> class gdb::ref_ptr'
     = gdb::ref_ptr<thread_info, refcounted_object_ref_policy>;
							     ^
  ../../src/gdb/gdbthread.h:376:59: note:   expected a type, got 'thread_info'
  ../../src/gdb/gdbthread.h:396:28: error: variable or field 'delete_thread' declared void
   extern void delete_thread (thread_info *thread);
			      ^
(...)

This is because there's a thread_info function in the Darwin/XNU/mach API:

 http://web.mit.edu/darwin/src/modules/xnu/osfmk/man/thread_info.html

Fix this in the same way it had been fixed in commit 7aabaf9d
("Create private_thread_info hierarchy"), by adding an explicit
"struct" keyword.

gdb/ChangeLog:
2018-06-25  Pedro Alves  <palves@redhat.com>

	* gdbthread.h (thread_info_ref, delete_thread)
	(delete_thread_silent, first_thread_of_inferior)
	(any_thread_of_inferior, switch_to_thread)
	(enable_thread_stack_temporaries)
	(thread_stack_temporaries_enabled_p, push_thread_stack_temporary)
	(get_last_thread_stack_temporary)
	(value_in_thread_stack_temporaries, can_access_registers_thread):
	Spell out "struct thread_info" instead of just "thread_info".
	* inferior.h (notice_new_inferior): Likewise.