help: Fix meson build with shell disabled

Bastien Nocerarequested to merge
wip/hadess/fix-meson-build into master 
$ meson -Dviewer=false -Dgtk_doc=true _build
<snip>
help/reference/shell/meson.build:21:6: ERROR:  Unknown variable "shell_inc".
Edited by Bastien Nocera

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