kbuild: confdata.c explicitly reference errno, thus need <errno.h>

This fixes:
% gmake LKC_GENPARSER=1 menuconfig
[...]
scripts/kconfig/confdata.c:739: error: 'errno' undeclared (first use in this function)
scripts/kconfig/confdata.c:739: error: (Each undeclared identifier is reported only once
scripts/kconfig/confdata.c:739: error: for each function it appears in.)
scripts/kconfig/confdata.c:739: error: 'ENOENT' undeclared (first use in this function)

triggered on NetBSD.

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>