Fix singleRX example to work correctly with LimeSDR Mini V2

The issue this commit fixes is that running singleRX example does
not show any samples streaming, and logs shows: RX data rate: 0 MB/s.

The fix follows the initialization order in basicRX.cpp where the
TX channel is enabled alongside with the RX channel right after
the LMS_Init(). With this fix running singleRX example shows samples
streamed: RX data rate: 32.1454 MB/s.

This is a bit counter-intuitive API from developers perspective (as
in, intuitively question is why TX channel is needed for RX-only
application). So it does feel like that perhaps the fix needs to be
done somewhere deeper in the driver or in gateware to increase
clarity of API. However, that'd be a deeper change, if it happens.
In any case, until such possible change the proposal is to fix the
issue in the same way as it is done in the other example.

The motivation to get the example fixed is because this is something
that new LimeSDR users run into as some of their first steps, so
having examples properly working solves initial confusion and possible
frustration.

src/examples/singleRX.cpp

@@ -54,6 +54,9 @@ int main(int argc, char** argv)
     if (LMS_Init(device) != 0)
         error();
 
+    if (LMS_EnableChannel(device, LMS_CH_TX, 0, true)!=0)	// Fix for v2
+        error();
+
     //Enable RX channel
     //Channels are numbered starting at 0
     if (LMS_EnableChannel(device, LMS_CH_RX, 0, true) != 0)