build: use codesign -f

Running 'make' twice currently results in an error:

```
% make
CGO_ENABLED=1 CGO_CFLAGS=-mmacosx-version-min=11.0 GOOS=darwin GOARCH=amd64 go build -o out/vfkit-amd64 ./cmd/vfkit
codesign --entitlements vf.entitlements -s - out/vfkit-amd64
out/vfkit-amd64: is already signed
make: *** [out/vfkit-amd64] Error 1
```

This commit makes use of "codesign -f" to force the re-generation of a
signature even if the binarie did not change. This works around the
previous error.

Makefile

@@ -18,7 +18,7 @@ clean:
 out/vfkit-amd64 out/vfkit-arm64: out/vfkit-%: force-build
  @mkdir -p $(@D)
  CGO_ENABLED=1 CGO_CFLAGS=$(CGO_CFLAGS) GOOS=darwin GOARCH=$* go build -o $@ ./cmd/vfkit
- codesign --entitlements vf.entitlements -s - $@
+ codesign -f --entitlements vf.entitlements -s - $@
 
 out/vfkit: out/vfkit-amd64 out/vfkit-arm64
  cd $(@D) && lipo -create $(^F) -output $(@F)