meson: use gir_init_section in GstTranscoder

meson: use gir_init_section in GstTranscoder

GstTranscoder adds extra_args for gir which call gst_init() during
introspection. These extra arguments are the same as the standard
ones defined in the top level meson.build as "git_init_section",
However, the top level definition also ensures an empty plugin
repository is used.

Because GstTranscoder does not use the standard args, plugins get
loaded when it is introspected. Since some of the plugins fail
without specific hardware, this causes #1100.

This patch makes it use gir_init_section.

Fixes #1100.