Fix fileutils not working on Windows

Change-Id: Ifcda643e35bcb63c3d7a235391ba95384f7ef444
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/53180
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

tools/src/fileutils/fileutils_other.go

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// +build !windows
+
 // Package fileutils contains utility functions for files
 package fileutils