Fix MSVC build with -DNONET=1

Not sure why this worked on Linux without this stub

SourceX/dvlnet/packet.h

@@ -34,6 +34,9 @@ typedef int turn_t;      // change int to something else in devilution code late
 typedef int leaveinfo_t; // also change later
 #ifndef NONET
 typedef std::array<unsigned char, crypto_secretbox_KEYBYTES> key_t;
+#else
+// Stub out the key_t defintion as we're not doing any encryption.
+using key_t = uint8_t;
 #endif
 
 static constexpr plr_t PLR_MASTER = 0xFE;