fix nipap/sql/Makefile

This should fix some problems in the Makefile.
I think we should actually move this to ip_net.plsql.

The reason it was not in that file in the first place is that pre-postgres 9.x there wasn't support for using the "CREATE EXTENSION x" syntax but the file had to be loaded manually from elsewhere. Now since we rely on 9.x, we might as well use the create-extension-syntax and do that in ip_net.plsql.