rust: fix bindgen build error with fstrict-flex-arrays

rust: fix bindgen build error with fstrict-flex-arrays

Commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3") enabled
'-fstrict-flex-arrays=3' globally, but bindgen does not recognized this
compiler option, triggering the following build error:

 error: unknown argument: '-fstrict-flex-arrays=3', err: true

[ Miguel: Commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
  did it so only conditionally (i.e. only if the C compiler supports
  it). This explains what Andrea was seeing: he was  compiling with a
  modern enough GCC, which enables the option, but with an old enough
  Clang. Andrea confirmed this was the case: he was using Clang 14 with
  GCC 13; and that Clang 15 worked for him.

  While it is possible to construct code (see mailing list for an
  example I came up with) where this could break, it is fairly
  contrived, and anyway GCC-built kernels with Rust enabled should
  only be used for experimentation until we get support for
  `rustc_codegen_gcc` and/or GCC Rust. So let's add this for the
  time being in case it helps somebody. ]

Add '-fstrict-flex-arrays' to the list of cflags that should be ignored
by bindgen.

Fixes: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Tested-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20230815065346.131387-1-andrea.righi@canonical.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>