support/kconfig: fix compile warning because of missing include.

support/kconfig: fix compile warning because of missing include.

With commit b58bf60b519353e983e372f5627d639e7cc72a7a the libgen.h
include was removed from confdata.c, but it is needed for the dirname
function declaration.

Fixes the following compile warning:
./confdata.c: In function ‘conf_split_config’:
./confdata.c:849:6: warning: assignment makes pointer from integer
without a cast

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>