scylla-gdb.py: register the scylla table command

This command exists but is not registered. There is a test for it, but
it happens to work only because scylla table is a prefix of scylla
tables (another command), so gdb invokes that other command instead.

scylla-gdb.py

@@ -6270,6 +6270,7 @@ scylla_databases()
 scylla_commitlog()
 scylla_keyspaces()
 scylla_tables()
+scylla_table()
 scylla_memory()
 scylla_ptr()
 scylla_mem_ranges()