include steam_api_flat.h (for ISteamUser_GetSteamID)

Index: src/CSteamworks.h
--- src/CSteamworks.h.orig
+++ src/CSteamworks.h
@@ -12,9 +12,11 @@ __pragma(warning(push))
 __pragma(warning(disable:4127)) // 'conditional expression is constant'
 __pragma(warning(disable:4996)) // 'strncpy': This function or variable may be unsafe. Consider using strncpy_s instead.
 #endif
+#define STEAMAPI_API extern "C"
 #include "steam_gameserver.h" // Includes steam_api internally
 #include "steamencryptedappticket.h"
 #include "isteamgamecoordinator.h"
+#include "steam_api_flat.h"
 #if defined( _WIN32 )
 __pragma(warning(pop))
 #endif
