applied patch #1912699 - "Pinned" / always loaded plugins support

this patch also adds plugin_api_version to exec_flags so your plug-in can now tell if features it needs are available
more plug-ins that need this will be converted once the patch to make both the stubs and the plug-ins use the same header file is in place

git-svn-id: https://svn.code.sf.net/p/nsis/code/NSIS/trunk@5809 212acab6-be3b-0410-9dea-997c60f758d6

Contrib/System/Source/Plugin.c

@@ -166,6 +166,11 @@ HANDLE GlobalCopy(HANDLE Old)
     return copymem(GlobalAlloc(GPTR, size), Old, (int) size);
 }
 
+UINT_PTR NSISCallback(UINT msg)
+{
+  return (UINT_PTR) NULL;
+}
+
 #ifdef _DEBUG
 void main()
 {