Common/DebugInterface: Use forward declarations where applicable

We're allowed (by the standard) to forward declare types within
std::vector, so we can replace direct includes with forward declarations
and then include the types where they're directly needed.

While we're at it, we can remove an unused inclusion of <cstring>, given
nothing in the header uses anything from it. This also revealed an
indirect inclusion, which this also resolves.

Source/Core/Core/HW/DSPLLE/DSPDebugInterface.h

@@ -8,6 +8,8 @@
 #include <string>
 
 #include "Common/CommonTypes.h"
+#include "Common/Debug/MemoryPatches.h"
+#include "Common/Debug/Watches.h"
 #include "Common/DebugInterface.h"
 
 namespace DSP::LLE