Logger: Append newlines to messages in error log files

Since 493e8f5 (Consistently use no LF in ERROR() calls) there are no
newlines in the messages, so they should be appended centrally.

Fixes SIPp#722.

src/logger.cpp

@@ -464,7 +464,7 @@ static void _screen_error(int fatal, bool use_errno, int error, const char *fmt,
     }
 
     if (error_lfi.fptr) {
-        count += fprintf(error_lfi.fptr, "%s", screen_last_error);
+        count += fprintf(error_lfi.fptr, "%s\n", screen_last_error);
         fflush(error_lfi.fptr);
         if (ringbuffer_size && count > ringbuffer_size) {
             rotate_errorf();