conformtest: Add alpha XFAIL for struct netent n_net type (bug 21260).

As noted in bug 21260, sysdeps/unix/sysv/linux/alpha/bits/netdb.h
defines struct netent with n_net of type unsigned long instead of the
correct uint32_t.  This patch XFAILs that test in the conformtest
expectations for alpha.  (This is not a fix for the bug, and it should
not be closed without an actual fix.)

Tested for alpha with build-many-glibcs.py.

	[BZ #21260]
	* sysdeps/unix/sysv/linux/alpha/Makefile
	[$(subdir) = conform] (conformtest-xfail-conds): New variable.