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

With commit b58bf60b 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>