[gnome-system-monitor] Add missing process priority page (fixes #161)

[Robert Roth <robertroth src gnome org>]

commit e7c92e3264e594adeeef483b86572423da0f39c1
Author: Robert Roth <robert roth off gmail com>
Date:   Sun Oct 18 10:36:23 2020 +0300

    Add missing process priority page (fixes #161)

 help/meson.build | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/help/meson.build b/help/meson.build
index d065bcc9..3e628b2d 100644
--- a/help/meson.build
+++ b/help/meson.build
@@ -22,6 +22,7 @@ gnome.yelp(meson.project_name(),
     'process-kill.page',
     'process-many.page',
     'process-priority-change.page',
+    'process-priority-what.page',
     'process-status.page',
     'process-update-speed.page',
     'solaris-mode.page',