x11/babl: Fix build
meson.build:1:0: ERROR: Value "false" (of type "string") for option "gi-docgen" is not one of the choices. Possible choices are (as string): "enabled", "disabled", "auto".
Approved by: blanket (fix build)
Fixes: 90274e2e7fe3 (update to 0.1.128 release)
Sponsored by: UNIS Labs