Fix compilation on OSX

Compilation fails on OSX with the following error:

~~~~
duplicate symbol _getdns_error in:
    build/temp.macosx-10.9-intel-2.7/getdns.o
    build/temp.macosx-10.9-intel-2.7/pygetdns_util.o
duplicate symbol _getdns_error in:
    build/temp.macosx-10.9-intel-2.7/getdns.o
    build/temp.macosx-10.9-intel-2.7/fd_poller.o
duplicate symbol _getdns_error in:
    build/temp.macosx-10.9-intel-2.7/getdns.o
    build/temp.macosx-10.9-intel-2.7/context.o
ld: 3 duplicate symbols for architecture x86_64
~~~~

This PR fixes it by making getdns.c the only compilation target.