I think it's the same thing discussed in this thread. Looking at WiFiClient.cpp I see the same send() used without using the parameter that would disable signal generation.
So the fix I've listed earlier should work. You say it doesn't? To make sure - you've modified line 161 of the WiFiClient.cpp, then recompiled and re-uploaded your sketch, haven't you? If that's correct, then just try the first fix, the one for the sketch itself. signal.h is not an Arduino library, but a standard Linux header, so you don't need to install it, it's already there.