Fix extra ; warnings involving MakeError

src/libexpr/json-to-value.hh

src/libexpr/nixexpr.hh

src/libstore/build.cc

src/libstore/store-api.hh

src/libutil/serialise.hh

src/libutil/thread-pool.hh

src/libutil/types.hh

src/libutil/util.hh