sigaddset: fix ARRAY_SIZE undefined

open_posix_testsuite/conformance/interfaces/sigaddset/4-1.c compiles failed
since b863a0bd, because of ARRAY_SIZE is undefined in open_posix_testsuite.

Signed-off-by: Zeng Linggang <zenglg.jy@cn.fujitsu.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>