remote.c: Use ptid_t instead of struct ptid

remote.c: Use ptid_t instead of struct ptid

It's the only place in the codebase that uses "struct ptid", so change
it to ptid_t for consistency.

gdb/ChangeLog:

	* remote.c (set_general_thread, set_continue_thread): Use ptid_t
	instead of struct ptid.