windows-nat.c: conflicting declaration of struct thread_info in C++ mode

windows-nat.c: conflicting declaration of struct thread_info in C++ mode

Building mingw GDB with --enable-build-with-cxx shows:

 ../../binutils-gdb/gdb/windows-nat.c: At global scope:
 ../../binutils-gdb/gdb/windows-nat.c:192:1: error: conflicting declaration 'typedef struct thread_info_struct thread_info'
  thread_info;
  ^
 In file included from ../../binutils-gdb/gdb/windows-nat.c:52:0:
 ../../binutils-gdb/gdb/gdbthread.h:160:8: error: 'struct thread_info' has a previous declaration as 'struct thread_info'
  struct thread_info
	 ^

Simply rename the structure to avoid the conflict.

gdb/ChangeLog:
2015-03-16  Yuanhui Zhang  <asmwarrior@gmail.com>
	    Pedro Alves  <palves@redhat.com>

	* windows-nat.c (struct thread_info_struct): Rename to ...
	(struct windows_thread_info_struct): ... this.
	(thread_info): Rename to ...
	(windows_thread_info): ... this.
	All users updated.