doc: Add since version for --device=input

Follow-up to flatpak#5481

Signed-off-by: Hubert Figuière <[email protected]>

doc/flatpak-metadata.xml

@@ -179,6 +179,7 @@
  <listitem><para>
  Input devices
  (<filename>/dev/input</filename>).
+ Available since 1.15.6.
  </para></listitem></varlistentry>
 
  <varlistentry><term><option>kvm</option></term>