Revert "build_wrapper: Add -prefix option for Windows builds"

Builds failed using the -prefix on Windows

This reverts commit 4c9a6cbd6d23d9ff88743c0538a02f92fe4c9467.

Change-Id: I01e294185dec26f00db10a096ece5bf50d9185d9
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>