Added support for Symbian OS platforms to recognize Nokia devices

User_agent class cannot recognize Nokia devices using Symbian OS returning the value Unknown Platform by adding these line it can be possible.

One of Agents string  tested:
Mozilla/5.0 (SymbianOS/9.4; U; Series60/5.0 Nokia5235-1d/12.8.092; Profile/MIDP-2.1 Configuration/CLDC-1.1 ) AppleWebKit/413 (KHTML, like Gecko) Safari/413