keymap: Add support for IBM ThinkPad X41 Tablet

Scancode taken from:
http://www.thinkwiki.org/wiki/Tablet_Hardware_Buttons#Linux_Support

William Jon McCann provided the DMI match. IBM seems to have
swapped the version and model of the system:
        Manufacturer: IBM
        Product Name: 18666TU
        Version: ThinkPad X41 Tablet
keymap: Fix special keys on ThinkPad X60/X61 Tablet

KEY_DIRECTION is mapped to XF86RotateWindows, to rotate the display:
http://cgit.freedesktop.org/xkeyboard-config/commit/symbols/inet?id=ec875f6f9b7c4028e11d32b071989c682e6502bd

And F13 is mapped to XF86Tools, which is closest to the original toolbox
usage:
http://cgit.freedesktop.org/xkeyboard-config/tree/symbols/inet?id=7a2c4bed212ebbcc05f3c959aef659ce7dd31fd8#n221