Hmmm, I won't swear to it but it looks suspiciously like a Ragweed seedling.
The young leaves can vary a bit in how they look but if it starts showing the typical ragweed leaves then rip it out!
Here's a seedling pic with leaves a tad younger than yours:
http://www.lsuagcenter.com/MCM...
Hopefully someone will chime in with a happier suggestion!
