Fixes break in mingw build

Postbuild step failed when building with mingw
diff --git a/Makefile b/Makefile
index 1283627..7f334ee 100644
--- a/Makefile
+++ b/Makefile
@@ -62,10 +62,11 @@
 
 # Post build steps
 post_build:
+ifndef WINDOWS
 	# If NULL Entropy is configured, display an appropriate warning
 	-scripts/config.pl get MBEDTLS_TEST_NULL_ENTROPY && ([ $$? -eq 0 ]) && \
 	    echo '$(NULL_ENTROPY_WARNING)'
-
+endif
 
 clean:
 	$(MAKE) -C library clean