The spice-rubbed, flame-grilled meat skewers known as suya aren’t something you normally find in restaurants in Nigeria. On the street corners and beaches of Lagos, a megacity of more than 20 million people, vendors—many from the Muslim-majority north—set wire grates up over hot coals, grill slices of beef, sprinkle them with a powdery spice mix that includes peanuts and chili, and wrap it all up in newspaper. Customers take their suya to go, sp…