combase: Add stub for RoGetServerActivatableClasses.

dlls/api-ms-win-core-winrt-registration-l1-1-0/api-ms-win-core-winrt-registration-l1-1-0.spec

 @ stub RoGetActivatableClassRegistration
-@ stub RoGetServerActivatableClasses
+@ stdcall RoGetServerActivatableClasses(ptr ptr ptr) combase.RoGetServerActivatableClasses

dlls/combase/combase.spec

@@ -254,7 +254,7 @@
 @ stub RoGetErrorReportingFlags
 @ stub RoGetMatchingRestrictedErrorInfo
 @ stdcall RoGetParameterizedTypeInstanceIID(long ptr ptr ptr ptr)
-@ stub RoGetServerActivatableClasses
+@ stdcall RoGetServerActivatableClasses(ptr ptr ptr)
 @ stdcall RoInitialize(long)
 @ stub RoInspectCapturedStackBackTrace
 @ stub RoInspectThreadErrorInfo

dlls/combase/roapi.c

@@ -251,3 +251,15 @@ HRESULT WINAPI RoRegisterForApartmentShutdown(IApartmentShutdown *callback,
         *cookie = (void *)0xcafecafe;
     return S_OK;
 }
+
+/***********************************************************************
+ *      RoGetServerActivatableClasses (combase.@)
+ */
+HRESULT WINAPI RoGetServerActivatableClasses(HSTRING name, HSTRING **classes, DWORD *count)
+{
+    FIXME("(%p, %p, %p): stub\n", name, classes, count);
+
+    if (count)
+        *count = 0;
+    return S_OK;
+}