[shotwell] build: Check for itstool

[Jens Georg <jensgeorg src gnome org>]

commit 556783991114d548ce272bdad90047c6cdb0b134
Author: Jens Georg <mail jensge org>
Date:   Fri Apr 3 20:16:48 2020 +0200

    build: Check for itstool
    
    The install target needs it for the yelp pages

 help/meson.build | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/help/meson.build b/help/meson.build
index 44ce9bfa..68dcba42 100644
--- a/help/meson.build
+++ b/help/meson.build
@@ -1,5 +1,7 @@
 install_man('shotwell.1')
 
+find_program('itstool', required: true)
+
 yelp_pages = [
     'edit-adjustments.page',
     'edit-crop.page',