update icu4c source location

* Needed after external/icu commit c73f511526464f8e56c242df80552e9b0d94ae3d
  "Move the icu4c source to match the upstream layout."

Change-Id: I3152e41d6979b8fb5735f08894f7ebdc2c5d4f18

Needed for building with AOSP master.