Fix mistaken changes in Makefile's clean target

I was a bit too trigger-happy with copy-pasting in a previous commit...
diff --git a/programs/Makefile b/programs/Makefile
index b23b11f..f62e2c6 100644
--- a/programs/Makefile
+++ b/programs/Makefile
@@ -267,7 +267,7 @@
 ifndef WINDOWS
 	rm -f $(APPS)
 else
-	del /Q /F *.c *.exe
+	del /S /Q /F *.o *.exe
 endif
 
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