checker: improve unused label check

vlib/builtin/builtin_windows.c.v

@@ -158,7 +158,7 @@ fn print_backtrace_skipping_top_frames_msvc(skipframes int) bool {
 					lnumber := sline64.f_line_number
 					lineinfo = '$file_name:$lnumber'
 				} else {
-					addr:
+					// addr:
 					lineinfo = '?? : address = 0x${(&frame_addr):x}'
 				}
 				sfunc := unsafe { tos3(fname) }

vlib/v/checker/checker.v

@@ -3915,6 +3915,7 @@ fn (mut c Checker) check_unused_labels() {
 		if nr_uses == 0 {
 			// TODO show label's location
 			c.warn('label `$label` defined and not used', token.Pos{})
+			c.goto_labels[label]++ // so that this warning is not shown again
 		}
 	}
 }