geoip: build tool should not rely on directory name

geoip: build tool should not rely on directory name

Fix this:

	GeoLite2-Country-CSV_20180905$ /usr/lib/xtables-addons/xt_geoip_build

	Use of uninitialized value $dir in concatenation (.) or string at
	/usr/lib/xtables-addons/xt_geoip_build line 59.
	Couldn't open list country names

Do not rely on any directory names (they change). Use the current
directory as the default source directory, similar to the older
xt_geoip_build (well, *.csv was passed as arguments).