Add missing requirement to Occupation of the Congo

Closes SighPie#158.

The mismatch between the decision requirements and the test performed in
the actual effects resulted in decision spam.

HFM/decisions/FlavourMod_Africa.txt

@@ -663,6 +663,7 @@ political_decisions = {
 					1980 = { empty = yes } #Congo
 				}
 				AND = {
+					invention = the_dark_continent
 					owns = 1981
 					1966 = { empty = yes } #Ubangi-Shari
 				}