Assume PROGMEM is available as soon as ARDUINO is defined (#1693)

extras/tests/Misc/weird_strcmp.hpp

@@ -1,5 +1,11 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright Benoit Blanchon 2014-2021
+// MIT License
+
 #include <ArduinoJson/Namespace.hpp>
 
+#include <string.h>  // strcmp, strncmp
+
 // Issue #1198: strcmp() implementation that returns a value larger than 8-bit
 
 namespace ARDUINOJSON_NAMESPACE {