mount-copybind: fix shellcheck warning

mount-copybind: fix shellcheck warning

$ shellcheck meta/recipes-core/volatile-binds/files/mount-copybind

In meta/recipes-core/volatile-binds/files/mount-copybind line 54:
            mountcontext=",rootcontext=$(matchpathcon -n $mountpoint)"
                                                         ^---------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean:
            mountcontext=",rootcontext=$(matchpathcon -n "$mountpoint")"

For more information:
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>