Added symbol for LF351 OpAmp

Created by: johnfwhitmore

I've been away from electronics for a while but back again and probably rusty and things might have changed in the process. This is more or less a direct copy of the LM741 in terms of the symbol and the pin designation as both are pretty much the same. I could possibly have created a new alias in the LM741, but I think the last time I did this, probably over a year ago, it was suggested to add new symbol. If that's wrong I can certainly change things.

Datasheet: http://www.farnell.com/datasheets/1880969.pdf