PathProbe doesn't look into sysroot

Use of PathProbe to find headers and libraries won't work with cross-compilers because the sysroot is not prepended to the paths.

Fixing this was as easy as defining a new import like this, but IMHO it should be done by default.

import qbs
import qbs.Probes

Probes.PathProbe {
  pathPrefixes: {
    if (qbs.sysroot !== undefined)
      return [qbs.sysroot + "/usr", qbs.sysroot + "/usr/local"];
    return [];
  }
}