shell-camera-monitor: Fix warning about strict aliasing

shell-camera-monitor: Fix warning about strict aliasing

Changing type when casting a double pointer is not strictly allowed in
C. In order to fix compilation with `-fstrict-aliasing`, don’t use
`g_clear_pointer()` for clearing this pointer where the type expected by
the destroy notify function does not match the type stored in the
`ShellCameraMonitor` struct.

This introduces no functional changes.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3351>