meson.build: set `LXC_DISTRO_SYSCONF` when `-Dspecfile=true`

meson.build: set `LXC_DISTRO_SYSCONF` when `-Dspecfile=true`

Before the change, the `setup` meson step would fail when disabling the
`install-init-files` option:

	$ meson setup -Dinstall-init-files=false build
	<snip>
	meson.build:936:44: ERROR: Entry LXC_DISTRO_SYSCONF not in configuration data.

This is because setting the `LXC_DISTRO_SYSCONF` option is conditional
and requires `install-init-files` to be enabled.

Meanwhile the `specfile` option (default enabled) also requires the
variable above, resulting in a failure when it is unset.

Amend the conditional to also set `LXC_DISTRO_SYSCONF` when `specfile`
option is `true`.

Fixes: 872db5424363 ("build: add more options for customizing install")
Signed-off-by: Roman Azarenko <roman.azarenko+gh@genexis.eu>