add fix to fuzz makefile for windows

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/programs/fuzz/Makefile b/programs/fuzz/Makefile
index 93dd4c9..50857ca 100644
--- a/programs/fuzz/Makefile
+++ b/programs/fuzz/Makefile
@@ -48,5 +48,5 @@
 else
 	if exist *.o del /Q /F *.o
 	if exist *.exe del /Q /F *.exe
-	rm -rf $(MBEDTLS_PATH)/tf-psa-crypto/programs/fuzz/*.o
+	if exist $(MBEDTLS_PATH)/tf-psa-crypto/programs/fuzz/*.o del /Q /F $(MBEDTLS_PATH)/tf-psa-crypto/programs/fuzz/*.o
 endif