Don't make MySQL-specific SQL call if MySQL isn't being used

I tried to get Bot running on a D7 installation using SQLite last night, but was getting PDO errors originating from this line in bot.smartirc.inc:

db_query('SET SESSION wait_timeout = 86400');

…because this is meaningless to SQLite, and probably other SQL engines as well. Fortunately, the workaround is simple, and after doing so, Bot seems to work just fine with SQLite (though I haven't tried it with any submodules yet, except my own).