Fix non pch jenkins tests build on Windows

With pch enabled WIN32 gets defined for cxxtests xmlformatter.h but not
without resulting in a build failure ctime_r not found which was only
added with C23.

Explicitly define WIN32 for Windows target, to force the intended
code path.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>

build/premake/extern_libs5.lua

@@ -260,6 +260,10 @@ extern_lib_defs = {
 				else
 					sysincludedirs { libraries_source_dir .. "cxxtest-4.4" }
 				end
+				-- Upstream uses WIN32 instead of _WIN32 define
+				if os.istarget("windows") then
+					defines { "WIN32" }
+				end
 			end
 		end,
 	},