oe-pkgdata-util: fix AttributeError

oe-pkgdata-util: fix AttributeError

The default arparse behaviour in Python 2 and early versions of Python 3
was to print usage message if no subcommands provided by user.
In Python >= 3.2.3 subparser has to have attribute 'required' set to
True to behave similar way.

By default subparser.required attribute is set to False, usage message is
not printed and code continues to be executed.

oe-pkgdata-util code raises AttributeError exception in
this case:
    AttributeError: 'Namespace' object has no attribute 'func'

Fixed this by setting subparser.required to True.

[YOCTO #9711]

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>