LONDON — It has been five years since Shirwa Ahmed, a 26-year-old from Minneapolis, blew himself up in northern Somalia, and sent shivers up American spines about young immigrants from war-torn Muslim countries who were turning into terrorists. Unconfirmed reports that American Muslims might have been among the gunmen who stormed Nairobi’s Westgate shopping mall over the weekend will no doubt revive fears of the “enemy within.”