shared random: missing field 'fetch_missing_votes' initializer

dgoulet's ticket16943_029_02 fails to compile on OS X with Apple LLVM version 7.3.0 (clang-703.0.31):

src/or/dirvote.c:2547:46: error: missing field 'fetch_missing_votes' initializer
      [-Werror,-Wmissing-field-initializers]

Even though this is perfectly legal C for "initialise the whole structure to zero".