gdbus: add dependency to gio-2.0

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gdbus: add rules for gdbus-codegen

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gdbus: convert modem-api.txt

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>