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/Debugger/PPCDebugInterface.h

@@ -7,6 +7,8 @@
 #include <cstddef>
 #include <string>
 
+#include "Common/Debug/MemoryPatches.h"
+#include "Common/Debug/Watches.h"
 #include "Common/DebugInterface.h"
 
 class PPCPatches : public Common::Debug::MemoryPatches