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

We use C99 features (fu-plugin-raspberrypi.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>

fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,vathpela/fwupd,fwupd/fwupd,vathpela/fwupd,superm1/fwupd,vathpela/fwupd,hughsie/fwupd,superm1/fwupd,vathpela/fwupd,hughsie/fwupd,superm1/fwupd,fwupd/fwupd,fwupd/fwupd