cmake: CMakeLists.txt: rename flat_mutation_reader.cc to readers/mutation_readers.cc
cmake: CMakeLists.txt: rename flat_mutation_reader.cc to readers/mutation_readers.cc

It was moved in 31d84a254c00b36dc2576e06ee288e28a13238195.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20220406110512.3731011-3-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla