__aligned is already defined on NetBSD.

__aligned is already defined on NetBSD.

Use __a_aligned in analogy to __a_unused which was introduced since __unused is
already defined on MacOS. This fixes the warning

	gcc-compat.h:5:1: warning: "__aligned" redefined
	In file included from /usr/include/regex.h:77,
			 from daemon.c:9:
	/usr/include/sys/cdefs.h:202:1: warning: this is the location of the previous definition

on NetBSD and has the additional advantage that it keeps working even if the
NetBSD people decide to redefine __aligned.