ovsdb-idlc.in: fix dict change during iteration.

ovsdb-idlc.in: fix dict change during iteration.

Python3 complains if a dict key is changed during the
iteration.

Use list() to create a copy of it.

Traceback (most recent call last):
  File "./ovsdb/ovsdb-idlc.in", line 1581, in <module>
    func(*args[1:])
  File "./ovsdb/ovsdb-idlc.in", line 185, in printCIDLHeader
    replace_cplusplus_keyword(schema)
  File "./ovsdb/ovsdb-idlc.in", line 179, in replace_cplusplus_keyword
    for columnName in table.columns:
RuntimeError: dictionary keys changed during iteration

Signed-off-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>