Try fixing compilation.

Include/RmlUi/Core/Types.h

@@ -61,6 +61,7 @@ namespace Core {
 using byte = unsigned char;
 using Time = double;
 using ScriptObject = void*;
+using std::size_t;
 
 // Unicode code point
 enum class Character : char32_t { Null, Replacement = 0xfffd };

Source/Core/Pool.h

@@ -32,6 +32,7 @@
 #include "../../Include/RmlUi/Core/Header.h"
 #include "../../Include/RmlUi/Core/Debug.h"
 #include "../../Include/RmlUi/Core/Traits.h"
+#include "../../Include/RmlUi/Core/Types.h"
 
 namespace Rml {
 namespace Core {