Fix unix2syscall_darwin.go build failure

Error was

 +GOOS=darwin
 +GOARCH=amd64
 +go build -tags without_openssl
 # github.com/rfjakob/gocryptfs/internal/syscallcompat
 internal/syscallcompat/unix2syscall_darwin.go:22:32: u.Atimespec undefined (type unix.Stat_t has no field or method Atimespec)
 internal/syscallcompat/unix2syscall_darwin.go:23:32: u.Mtimespec undefined (type unix.Stat_t has no field or method Mtimespec)
 internal/syscallcompat/unix2syscall_darwin.go:24:32: u.Ctimespec undefined (type unix.Stat_t has no field or method Ctimespec)

caused by https://github.com/golang/sys/commit/87c872767d25fb96dfe96c794fd028b38a08440b#diff-4913a9178621eadcdf191db17915fbcb

1 files changed, 2 insertions, 2 deletions

diff --git a/Gopkg.lock b/Gopkg.lock
index eeb62f2..d33573c 100644
--- a/Gopkg.lock
+++ b/Gopkg.lock
@@ -75,14 +75,14 @@
 
 [[projects]]
   branch = "master"
-  digest = "1:ff0bd364800b7ec504f142151d0c2fbe68841cd1dc1a506fae10eeddcae42e60"
+  digest = "1:677902b7cbbfa124deba62fc56aaa9ec7ad76ab7c45d42f76c973d3e7a1ef109"
   name = "golang.org/x/sys"
   packages = [
     "unix",
     "windows",
   ]
   pruneopts = ""
-  revision = "6c81ef8f67ca3f42fc9cd71dfbd5f35b0c4b5771"
+  revision = "61b9204099cb1bebc803c9ffb9b2d3acd9d457d9"
 
 [solve-meta]
   analyzer-name = "dep"