build: move ipset_errcode into library

build: move ipset_errcode into library

The library cannot stand on its own:

19:13 seven:../ipset/lib > ldd -r .libs/libipset.so.1
        linux-vdso.so.1 =>  (0x00007fff9a569000)
        libmnl.so.0 => /usr/lib64/libmnl.so.0 (0x00007fd42ae5c000)
        libc.so.6 => /lib64/libc.so.6 (0x00007fd42aaef000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fd42b28d000)
undefined symbol: ipset_errcode (.libs/libipset.so.1)

Resolve this by moving ipset_errcode into the library.

Reported-by: Arkadiusz Miskiewicz <a.miskiewicz@gmail.com>
References: http://marc.info/?l=netfilter-devel&m=131435791514602&w=2