Make "get_field_default_value" a non "static" function

To implement "Create Table <table-name> LIKE <existing-table-name>" statement in the ColumnStore engine it needs to re-create the CREATE TABLE statement and process it in the DDL processing code of the ColumnStore. An easy way to do this is to iterate over the list of column fields and create appropriate SQL parts for each column. sql_show.cc contains a nice function get_field_default_values() that decodes the Field and other associated data structures to create the default value SQL string. Currently this function is static so cannot be called from anywhere else. Therefore if it can be made non-static, code duplication can be avoided.