implement message folder base Compact() method

mailnews/base/util/nsMsgFolder.cpp

@@ -633,6 +633,10 @@ NS_IMETHODIMP nsMsgFolder::CreateSubfolder(const char *folderName)
 
 }
 
+NS_IMETHODIMP nsMsgFolder::Compact()
+{
+  return NS_OK;
+}
 
 NS_IMETHODIMP nsMsgFolder::Rename(const char *name)
 {

mailnews/base/util/nsMsgFolder.h

@@ -136,6 +136,8 @@ public:
 
   NS_IMETHOD CreateSubfolder(const char *folderName);
 
+  NS_IMETHOD Compact();
+
   NS_IMETHOD Rename(const char *name);
   NS_IMETHOD Adopt(nsIMsgFolder *srcFolder, PRUint32*);