contrib: libarchive: fix the missing statvfs patch

The code assumed that statfs had a f_flag field in some places (but not others)
but it's in fact it's f_flags.

Android compiles fine with this patch (and all other targets in Gitlab CI)
12 jobs for !8 with outdated-libarchive-statvfs in 91 minutes and 19 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Build
passed #378071
amd64 docker
android-arm

00:17:06

passed #378072
amd64 docker
android-arm64

00:17:25

passed #378073
amd64 docker
android-x86

00:11:53

passed #378074
amd64 docker
android-x86_64

00:16:42

passed #378066
amd64 docker
debian

00:18:29

passed #378070
macos
ios-arm64

00:37:07

passed #378069
macos
macos

00:46:16

passed #378068
amd64 docker
raspbian-arm

00:13:26

passed #378067
amd64 docker
snap

00:35:58

passed #378063
amd64 docker
win32

00:18:50

passed #378064
amd64 docker
win64

00:29:22

passed #378065
amd64 docker
win64-llvm

00:31:41