[gnome-system-monitor] meson: check for necessary tools

[Robert Roth <robertroth src gnome org>]

commit addd6db6aff45df59851238e5032a388a0f67b66
Author: Günther Wagner <info gunibert de>
Date:   Fri Jul 16 22:40:54 2021 +0200

    meson: check for necessary tools

 help/meson.build | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/help/meson.build b/help/meson.build
index 4ec6a297..9abfcdd2 100644
--- a/help/meson.build
+++ b/help/meson.build
@@ -1,3 +1,4 @@
+find_program('itstool', required: true)
 gnome.yelp(meson.project_name(),
   sources: [
     'commandline.page',