useradd.bbclass: add dependency on base-files

useradd.bbclass: add dependency on base-files

Packages that use useradd.bbclass should have a dependency on base-files so that
the /etc/skel directory is populated.  Without this dependency base-files may or
may not be installed when the postinst runs, and the skel content may or may not
be copied.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>