build: flowtop: Only build ioops with GeoIP support enabled

Follow commit 9ad7f8882097 ("build: astraceroute: Only build ioops with
GeoIP support enabled").

The ioops module is only needed in geoip.o, thus make it dependent on
CONFIG_GEOIP.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>