multimedia/gstreamer1-vaapi: unbreak with --auto-features=enabled

meson.build:93:0: ERROR: Dependency "libudev" not found, tried pkgconfig
meson.build:120:0: ERROR: Dependency "gtk+-3.0" not found, tried pkgconfig
docs/meson.build:42:0: ERROR: Program 'hotdoc' not found or not executable

PR: 270361
Reported by: antoine (via exp-run)