net-mgmt/pmacct: Unbreak build with --enable-option-checking=fatal

> Configuring for pmacct-1.7.5

configure: error: unrecognized options: --disable-, --enable-64bit

There is no --enable-64bit option anymore. The --disable- is caused
by the extra = after KAFKA_CONFIGURE_ENABLE= because the framework
splits *_CONFIGURE_ENABLE on =. Arguably it should raise an error
here instead of appending nonsense like --disable-. This took
forever to find. :-(

PR: 255291