coccinelle: misc: add uninitialized_var.cocci script

uninitialized_var() macro was removed from the sources [1] and
other warning-silencing tricks were deprecated [2]. The purpose of this
cocci script is to prevent new occurrences of uninitialized_var()
open-coded variants.

[1] commit 63a0895d ("compiler: Remove uninitialized_var() macro")
[2] commit 4b19bec9 ("docs: deprecated.rst: Add uninitialized_var()")

Cc: Kees Cook <keescook@chromium.org>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>