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

@@ -189,7 +189,6 @@ warning_c_flags = [
   '-Wmissing-prototypes',
   '-Wdeclaration-after-statement',
   '-Wold-style-definition',
-  '-Wnested-externs',
 ]
 
 warning_cxx_flags = [