Added missing include directives.

diff --git a/include/guiddef.h b/include/guiddef.h
index 9f1b7d8..e91ce2c 100644
--- a/include/guiddef.h
+++ b/include/guiddef.h
@@ -52,6 +52,7 @@
 #define IsEqualCLSID(rclsid1, rclsid2) IsEqualGUID(rclsid1, rclsid2)
 
 #if defined(__cplusplus) && !defined(CINTERFACE)
+#include <string.h>
 inline bool operator==(const GUID& guidOne, const GUID& guidOther)
 {
     return !memcmp(&guidOne,&guidOther,sizeof(GUID));