Add IInputSystem and IInputInternal interfaces.

Signed-off-by: aixxe <me@aixxe.net>

include/cstrike/Interfaces/IInputSystem.h

@@ -0,0 +1,10 @@
+#pragma once
+
+enum ButtonCode_t;
+
+class IInputSystem {
+	public:
+		bool IsButtonDown(ButtonCode_t button) {
+			return GetVirtualFunction<bool(*)(void*, ButtonCode_t)>(this, 11)(this, button);
+		}
+};