hackrf_transfer: send signal to own PID

Instead of sending a signal to the process group, get our own process ID
and send the signal to it. This fixes a bug that prevented termination
when called from a script.

host/hackrf-tools/src/hackrf_transfer.c

@@ -397,7 +397,7 @@ void stop_main_loop(void)
 #ifdef _WIN32
 	SetEvent(interrupt_handle);
 #else
-	kill(0, SIGALRM);
+	kill(getpid(), SIGALRM);
 #endif
 }