The share of U.S. workers in unions last year was unchanged at 11.1%, according to Labor Department figures released Thursday, a reflection of the labor movement’s struggle to reverse years of stagnation that followed decades of declines. . . Membership in the private sector rose slightly to 6.7% from 6.6% in 2014 as unions added 195,000 more members there. The public sector had 23,000 more unionized workers in 2015 than it did the year before, but the membership rate fell to 35.2% from 35.7% over that period due to a larger increase in nonunion government workers.