GCMemcard: Required declaration before C++17.

Source/Core/Core/HW/GCMemcard/GCMemcard.cpp

@@ -20,6 +20,9 @@
 #include "Common/StringUtil.h"
 #include "Common/Swap.h"
 
+// TODO: Remove when on C++17 everywhere.
+constexpr std::array<u8, 4> DEntry::UNINITIALIZED_GAMECODE;
+
 static void ByteSwap(u8* valueA, u8* valueB)
 {
   u8 tmp = *valueA;