conformtest: Allow *_t in sys/socket.h.

The conformtest expectations in sys/socket.h are missing the standard
POSIX allowance for all headers to define *_t names.  For XSI standard
versions that allowance comes in via the permission to include
<sys/uio.h> (for which the expectations properly allow *_t), but for
non-XSI POSIX nothing brings in that allowance and spurious namespace
failures occur.  This patch adds the required permission to the
expectations to remove the spurious failures (by itself this does not
allow any XFAILs to be removed).

Tested for x86_64.

	* conform/data/sys/socket.h-data (*_t): Allow.