Added extern "C" if __cplusplus is defined.

include/dplay.h

diff --git a/include/dplay.h b/include/dplay.h
index 9445d70..54d359b 100644
--- a/include/dplay.h
+++ b/include/dplay.h
@@ -3,6 +3,10 @@
 
 #include "wine/obj_base.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif /* defined(__cplusplus) */
+
 #include "pshpack1.h"
 
 /*****************************************************************************
@@ -585,4 +589,8 @@
 #define IDirectPlay3_GetPlayerFlags(p,a,b)                 ICOM_CALL2(GetPlayerFlags,p,a,b)
 #endif
 
+#ifdef __cplusplus
+} /* extern "C" */
+#endif /* defined(__cplusplus) */
+
 #endif /* __WINE_DPLAY_H */