row_cache: Fix compilation errors with gcc 5
row_cache: Fix compilation errors with gcc 5

Message-Id: <beba93470e78ce445228ccd198da94ae3abb3dd1@scylladb.com>

scylladb/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla