panels: Fix build with NetworkManager disabled

Gentoo allows the user to disable NetworkManager integration at build time.
We use the existing HAVE_NETWORK_MANAGER preprocessor macro to guard
the uses of the mobile_switch_changed_cb and wifi_switch_changed_cb
functions that guard their definitions.

Bug: https://bugs.gentoo.org/783144
Closes: #1335
7 jobs for gnome-40 in 20 seconds
latest
Status Job ID Name Coverage
  Build
failed #1311445
build

00:00:20

 
  Test
skipped #1311446
test
 
  Manual
skipped #1311448
asan allowed to fail manual
asan
skipped #1311447
allowed to fail manual
flatpak
skipped #1311451
allowed to fail manual
msan
skipped #1311449
allowed to fail manual
tsan
skipped #1311450
allowed to fail manual
ubsan
 
Name Stage Failure
failed
build Build

A full log can be found at /builds/maxbritov/gnome-control-center/_build/meson-logs/meson-log.txt
$ echo "== Installing =="
== Installing ==
$ ninja -C _build install
ninja: Entering directory `_build'
ninja: error: loading 'build.ninja': No such file or directory
Cleaning up file based variables
ERROR: Job failed: exit code 1