help: Fix meson build with shell disabled
$ meson -Dviewer=false -Dgtk_doc=true _build
<snip>
help/reference/shell/meson.build:21:6: ERROR: Unknown variable "shell_inc".
Edited by Bastien Nocera
$ meson -Dviewer=false -Dgtk_doc=true _build
<snip>
help/reference/shell/meson.build:21:6: ERROR: Unknown variable "shell_inc".