One way is to use custom plugin. In fact, such custom plugin doesn't need to do anything, only declare new toolbar contribution using existing undo/redo commands.
I've built such plugin for you: http://www.foglyn.com/misc/undoredo_1.0.0.jar. There is absolutely no code, only plugin.xml:
<?xml version="1.0" encoding="UTF-8"?>
<?eclipse version="3.4"?>
<plugin>
<extension
point="org.eclipse.ui.menus">
<menuContribution
locationURI="toolbar:org.eclipse.ui.main.toolbar?after=additions">
<toolbar
id="undoredo.toolbar">
<command
commandId="org.eclipse.ui.edit.undo"
id="undoredo.undo"
style="push">
</command>
<command
commandId="org.eclipse.ui.edit.redo"
id="undoredo.redo"
style="push">
</command>
</toolbar>
</menuContribution>
</extension>
</plugin>
And MANIFEST.MF:
Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: Undoredo
Bundle-SymbolicName: undoredo;singleton:=true
Bundle-Version: 1.0.0
Bundle-RequiredExecutionEnvironment: J2SE-1.5
Require-Bundle: org.eclipse.ui
You can download it, and drop into your 'dropins' directory of Eclipse, restart, and you'll see Undo/Redo buttons on your toolbar.
Works for me in Eclipse 3.4 and Eclipse 3.5M7.