package/eudev: fix build with kernel < 4.8

package/eudev: fix build with kernel < 4.8

KEY_ALS_TOGGLE has been added in kernel 4.8 with
https://github.com/torvalds/linux/commit/9ee27487127461b5cf71670b708ed5b2b8da568c

So add it to missing.h to fix build with kernel older than 4.8

Fixes:
 - http://autobuild.buildroot.org/results/0c0aff02df91cdb869efa01e397f7ccc0d9f69af

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Matthew Weber <matthew.weber@rockwellcollins.con>
[Thomas: update upstream status.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>