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>
diff --git a/scripts/kconfig/confdata.c b/scripts/kconfig/confdata.c
index 515253f..dc11d51 100644
--- a/scripts/kconfig/confdata.c
+++ b/scripts/kconfig/confdata.c
@@ -5,6 +5,7 @@
 
 #include <sys/stat.h>
 #include <ctype.h>
+#include <errno.h>
 #include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>