ccid_host/Makefile: Ensure 'make clean' succeeds (rm -f)

Change-Id: Ie4e1fa64b707d86a20582b1b88612e5d268db558
diff --git a/ccid_host/Makefile b/ccid_host/Makefile
index b68bd9d..53ee932 100644
--- a/ccid_host/Makefile
+++ b/ccid_host/Makefile
@@ -13,4 +13,4 @@
 	$(CC) $(CFLAGS) -o $@ -c $^
 
 clean:
-	rm ccid_functionfs hub_functionfs *.o
+	rm -f ccid_functionfs hub_functionfs *.o