build: Enable C99 support in meson.build
build: Enable C99 support in meson.build

We use C99 features (various places in as-app-validate.c), so need to
explicitly enable them in meson.build, as some compilers will not enable
them automatically, and will instead error when they encounter usage of
C99.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

ximion/appstream-glib,ximion/appstream-glib,ximion/appstream-glib,hughsie/appstream-glib,hughsie/appstream-glib,hughsie/appstream-glib