Move to #pragma warning to fix MSVC 2019 compatibility

This isn't fully portable until MSVC 2022.

Change-Id: I4a4d50e10a13c943c30bca23dc546612c516c8d2
Reviewed-on: https://codereview.kdab.com/c/kdab/kdutils/+/137071
Tested-by: Continuous Integration <build@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>

src/KDFoundation/logging.h

@@ -12,7 +12,7 @@
 // TODO: Remove once downstream libs are ported to use KDUtils::Logger
 #pragma once
 
-#warning "This header is deprecated, logging.h is now under <KDUtils/logging.h>"
+#pragma warning("This header is deprecated, logging.h is now under <KDUtils/logging.h>")
 
 #include <spdlog/spdlog.h>
 #include <spdlog/sinks/stdout_color_sinks.h>