Fixed compilation on non-Windows OSes

src/posix/i_system.cpp

@@ -758,3 +758,9 @@ unsigned int I_MakeRNGSeed()
 	}
 	return seed;
 }
+
+TArray<FString> I_GetGogPaths()
+{
+    // GOG's Doom games are Windows only at the moment
+    return TArray<FString>();
+}

src/posix/i_system.h

@@ -124,6 +124,8 @@ int I_PickIWad (WadStuff *wads, int numwads, bool queryiwad, int defaultiwad);
 // directories for IWADs if the user purchased any through Steam.
 TArray<FString> I_GetSteamPath();
 
+TArray<FString> I_GetGogPaths();
+
 // The ini could not be saved at exit
 bool I_WriteIniFailed ();