Added extern "C" if __cplusplus is defined.

diff --git a/include/msacm.h b/include/msacm.h
index 431845c..9b24586 100644
--- a/include/msacm.h
+++ b/include/msacm.h
@@ -10,6 +10,10 @@
 #include "mmsystem.h"
 #include "mmreg.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif /* defined(__cplusplus) */
+
 /***********************************************************************
  * Defines/Enums
  */
@@ -977,6 +981,10 @@
 PWINE_ACMOBJ MSACM_GetObj(HACMOBJ hObj);
 HDRVR MSACM_OpenDriverProc(DRIVERPROC pfnDriverProc);
 
+#ifdef __cplusplus
+} /* extern "C" */
+#endif /* defined(__cplusplus) */
+
 #endif  /* __WINE_MSACM_H */