gunixmounts: Stop considering cifs/nfs as system file systems

They’re network file systems, but not system file systems (in the sense
that procfs is a system file system). This fixes them disappearing from
the sidebar in the UI.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

GNOME/glib#1424
6 jobs for cifs-is-not-a-system-fs in 44 minutes and 5 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #50775
fedora-meson-android_ndk_r16_api21_arm64

00:41:49

passed #50776
fedora-meson-mingw64

00:42:52

passed #50774
fedora-meson-x86_64

00:15:48

passed #50777
win32
msys2-mingw32

00:08:36

passed #50778
win32
vs2017-x64

00:05:37

 
  Coverage
passed #50779
coverage

00:01:10

76.9%