[bootlin/training-materials] 9103e0: Makefile: check for all labs slide to exists

[Clément Léger]

  Branch: refs/heads/master
  Home:   https://github.com/bootlin/training-materials
  Commit: 9103e0af579c069c0d230403743b3cb2d7ffcd50
      https://github.com/bootlin/training-materials/commit/9103e0af579c069c0d230403743b3cb2d7ffcd50
  Author: Clément Léger <clement.leger at bootlin.com>
  Date:   2022-05-23 (Mon, 23 May 2022)

  Changed paths:
    M Makefile

  Log Message:
  -----------
  Makefile: check for all labs slide to exists

Commit 292167ee393 ("Makefile: check for all .tex files to exist") did
add a check for all slides to exists but this was not done for the
labs. add the same check.

Signed-off-by: Clément Léger <clement.leger at bootlin.com>