Fix alpha termios.h NL2, NL3 namespace (bug 21268).

sysdeps/unix/sysv/linux/alpha/bits/termios.h defines NL2 and NL3 for
__USE_MISC || __USE_XOPEN.  These should only be defined for
__USE_MISC as they are not part of any standard namespace.  This patch
conditions them accordingly, matching the powerpc version of the
header (the only other one in glibc that defines these macros).

Tested (compilation only) with build-many-glibcs.py.

	[BZ #21268]
	* sysdeps/unix/sysv/linux/alpha/bits/termios.h (NL2): Define only
	if [__USE_MISC]
	(NL3): Likewise.