ci: Add --help option to meson-install script

It's not really necessary in an internal script, but it's easy
enough and doesn't hurt *shrug*

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2712>

.gitlab-ci/install-meson-project.sh

@@ -11,12 +11,15 @@ usage() {
 	Options:
 	  -Dkey=val      Option to pass on to meson
 
+	  -h, --help     Display this help
+
 	EOF
 }
 
 TEMP=$(getopt \
   --name=$(basename $0) \
-  --options=D \
+  --options='D:h' \
+  --longoptions='help' \
   -- "$@")
 
 eval set -- "$TEMP"
@@ -31,6 +34,11 @@ while true; do
       shift 2
     ;;
 
+    -h|--help)
+      usage
+      exit 0
+    ;;
+
     --)
       shift
       break