build: syscalls: Add unmount flags.

* guix/build/syscalls.scm (MNT_FORCE, MNT_DETACH, MNT_EXPIRE)
  (UMOUNT_NOFOLLOW): New variables.

guix/build/syscalls.scm

@@ -34,6 +34,10 @@ (define-module (guix build syscalls)
             MS_BIND
             MS_MOVE
             MS_STRICTATIME
+            MNT_FORCE
+            MNT_DETACH
+            MNT_EXPIRE
+            UMOUNT_NOFOLLOW
             restart-on-EINTR
             mount
             umount
@@ -150,6 +154,11 @@ (define MS_BIND            4096)
 (define MS_MOVE            8192)
 (define MS_STRICTATIME 16777216)
 
+(define MNT_FORCE       1)
+(define MNT_DETACH      2)
+(define MNT_EXPIRE      4)
+(define UMOUNT_NOFOLLOW 8)
+
 (define mount
   (let* ((ptr  (dynamic-func "mount" (dynamic-link)))
          (proc (pointer->procedure int ptr `(* * * ,unsigned-long *))))