gdbserver/tracepoint: Add 'struct sockaddr_un *' -> 'struct sockaddr *' cast

gdbserver/tracepoint: Add 'struct sockaddr_un *' -> 'struct sockaddr *' cast

Fixes this in C++ mode:

  gdb/gdbserver/tracepoint.c: In function ‘void* gdb_agent_helper_thread(void*)’:
  gdb/gdbserver/tracepoint.c:7190:47: error: cannot convert ‘sockaddr_un*’ to ‘sockaddr*’ for argument ‘2’ to ‘int accept(int, sockaddr*, socklen_t*)’
	  fd = accept (listen_fd, &sockaddr, &tmp);

gdb/gdbserver/ChangeLog:
2015-03-09  Pedro Alves  <palves@redhat.com>

	* tracepoint.c (gdb_agent_helper_thread): Cast '&sockaddr' to
	'struct sockaddr' pointer in 'accept' call.