net: buf: remove gcc dependency

Placing a struct with a flexible array member inside another struct not
as the last member is a GCC only extension. The offending macro is only
used to allocate enough space sufficient for a `struct net_buf`.
Therefore the dummy type declaration can simply use a byte array of the
same size as a `struct net_buf`, instead of the `struct net_buf` itself.

Fixes #42586.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>