g-i: silence 'nested extern' compiler warnings when building scanner binary

We need a nested extern in our init section for the scanner binary
so we can call gst_init to make sure GStreamer types are initialised
(they are not all lazy init via get_type functions, but some are in
exported variables). There doesn't seem to be any other mechanism to
achieve this, so just remove that warning, it's not important at all.

meson.build

@@ -118,7 +118,6 @@ warning_flags = [
   '-Wformat-security',
   '-Wold-style-definition',
   '-Waggregate-return',
-  '-Wnested-externs',
   '-Winit-self',
   '-Wmissing-include-dirs',
   '-Waddress',