JANITORIAL: Reduce header dependencies in shared code

Some backends may break as I only compiled SDL

base/plugins.h

@@ -27,16 +27,10 @@
 #define BASE_PLUGINS_H
 
 #include "common/array.h"
-#include "common/error.h"
-#include "common/singleton.h"
-#include "common/util.h"
+#include "common/fs.h"
+#include "common/str.h"
 #include "backends/plugins/elf/version.h"
 
-namespace Common {
-class FSList;
-class FSNode;
-}
-
 
 /**
  * @page pagePlugins An overview of the ScummVM plugin system