Put this into your ~/.gtkrc-2.0
and you should be good to go. The Left and Right lines make the requested change, the rest are just my personal additions to make the tree-view act more vim-like.
binding "gtk-binding-tree-view" {
bind "j" { "move-cursor" (display-lines, 1) }
bind "k" { "move-cursor" (display-lines, -1) }
bind "h" { "expand-collapse-cursor-row" (1,0,0) }
bind "l" { "expand-collapse-cursor-row" (1,1,0) }
bind "o" { "move-cursor" (pages, 1) }
bind "u" { "move-cursor" (pages, -1) }
bind "g" { "move-cursor" (buffer-ends, -1) }
bind "y" { "move-cursor" (buffer-ends, 1) }
bind "p" { "select-cursor-parent" () }
bind "Left" { "expand-collapse-cursor-row" (0,0,0) }
bind "Right" { "expand-collapse-cursor-row" (0,1,0) }
bind "semicolon" { "expand-collapse-cursor-row" (0,1,1) }
bind "slash" { "start-interactive-search" () }
}
class "GtkTreeView" binding "gtk-binding-tree-view"
then restart your Eclipse to apply new bindings