package/python-iptables: account for platform tag in extensions

package/python-iptables: account for platform tag in extensions

EXT_SUFFIX in Python versions > 3.5 contains a platform tag which only applies
to cpython extensions. Given that ctypes.util.find_library does not work on the
target due to the absence of the underlying tools '.so' needs to be added as a
possible suffix for libraries to enable python-iptables to find the iptables
shared libraries.

Signed-off-by: Frank Vanbever <frank.vanbever@essensium.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>