Attempt to fix build by a workaround for a bug in Windows8.1 SDK

Problem description and workaround are here
https://developercommunity.visualstudio.com/content/problem/185399/error-c2760-in-combaseapih-with-windows-sdk-81-and.html

sql/mysql_install_db.cc

@@ -26,6 +26,7 @@
 #include <shellapi.h>
 #include <accctrl.h>
 #include <aclapi.h>
+struct IUnknown;
 #include <shlwapi.h>
 
 #define USAGETEXT \