build: avoid meson error when disabling gtk
Previously, when building with meson _build -D gtk=no, I got:
src/meson.build:25:11: ERROR: Unknown method "found" in object <[ArrayHolder] holds [list]: []> of type ArrayHolder.
This has already been addressed in meson by pr#2615:
https://github.com/mesonbuild/meson/pull/2615