extensions: link on libxtables and check symbols

extensions: link on libxtables and check symbols

Have each extension link against libxtables.so; with this, all home
symbols are known at link time and we can use ld's --no-undefined to
run the check, dropping the homebrew solution.

By having libxtables.so required by extensions, package managers'
automatic dependency discovery will become effective so that manual
dependencies for distros with split extension packages (e.g. OpenWRT)
will not be necessary anymore.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>