iptables: optional loading of static extensions

iptables: optional loading of static extensions

Make loading of static extensions optional to support vanilla iptables
in it's default configuration by setting DISABLE_STATIC_EXTENSIONS
instead of hackery.

In case iptables is built with --disable-static libext.a, libext4.a and
libext6.a which OpenWrt installs in the form of libiptext.so,
libiptext4.so, libiptext6.so to save a couple more bytes are of no use
or non-existent one could say. So this commit avoids requiring a
tampered with iptables.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
[Jo-Philipp Wich: stub init_extensions*() instead to reduce amount of ifdefs]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>