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

Source/exehead/SConscript

@@ -4,6 +4,7 @@ files = Split("""
 	exec.c
 	fileform.c
 	Main.c
+	plugin.c
 	Ui.c
 	util.c
 	#Source/crc32.c