cpu/arm/k60: Add missing end of comment.

Adds a missing '*/', fix warning:

    ../../contiki/cpu/arm/k60/./rtimer-arch.c:115:3: warning: '/*' within block comment [-Wcomment]

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
Merge pull request #33 from gebart/for-mulle/rtimer-close-comment

cpu/arm/k60: Add missing end of comment.