main returns 0 upon normal ternimation

The return value for main should indicate how the program exited. Normal
exit is generally represented by a 0 return value from main. Abnormal
termination is usually signalled by a non-zero return but there is no
standard for how non-zero codes are interpreted

src/main/main.cc

@@ -224,4 +224,5 @@ int main(int argc, char** argv)
 
     google::ShutDownCommandLineFlags();
     std::cout << "GNSS-SDR program ended." << std::endl;
+    return 0;
 }