Make sql_alloc() declaration "public" for a client context.
The reason is that sql_alloc() is used in definition of some common
purpose stuff (e.g. sql_list.*). To make this stuff available for a
client context we declare sql_alloc() as a "virtual function", i.e.
as a function that is already declared but must be defined in this
context (note that definition of sql_alloc() in thr_malloc.cc is
#ifndef'ed for MYSQL_CLIENT).
Also make sql_string.h repeatedly includable.