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/PowerPC/PPCSymbolDB.cpp

@@ -5,6 +5,7 @@
 #include "Core/PowerPC/PPCSymbolDB.h"
 
 #include <algorithm>
+#include <cstring>
 #include <map>
 #include <string>
 #include <utility>