libffi: backport fix for implicit function declarations in libffi

On `clang-16` it's a build failure. On `gcc` it's a prototype mismatch
with a potential invalid runtime results:

    ../src/tramp.c:262:22: warning: implicit declaration of function 'open_temp_exec_file' [-Wimplicit-function-declaration]

pkgs/development/libraries/libffi/default.nix

@@ -22,6 +22,9 @@ stdenv.mkDerivation rec {
   # cgit) that are needed here should be included directly in Nixpkgs as
   # files.
   patches = [
+    # Fix implicit function declarations (clang-16 build failure):
+    #     https://github.com/libffi/libffi/pull/764
+    ./fix-implicit-fun-decl.patch
   ];
 
   strictDeps = true;

pkgs/development/libraries/libffi/fix-implicit-fun-decl.patch

@@ -0,0 +1,46 @@
+https://github.com/libffi/libffi/commit/ce077e5565366171aa1b4438749b0922fce887a4.patch
+
+From ce077e5565366171aa1b4438749b0922fce887a4 Mon Sep 17 00:00:00 2001
+From: serge-sans-paille <serge.guelton@telecom-bretagne.eu>
+Date: Thu, 2 Feb 2023 14:46:29 +0000
+Subject: [PATCH] Forward declare open_temp_exec_file (#764)
+
+It's defined in closures.c and used in tramp.c.
+Also declare it as an hidden symbol, as it should be.
+
+Co-authored-by: serge-sans-paille <sguelton@mozilla.com>
+---
+ include/ffi_common.h | 4 ++++
+ src/tramp.c          | 4 ++++
+ 2 files changed, 8 insertions(+)
+
+diff --git a/include/ffi_common.h b/include/ffi_common.h
+index 2bd31b03d..c53a79493 100644
+--- a/include/ffi_common.h
++++ b/include/ffi_common.h
+@@ -128,6 +128,10 @@ void *ffi_data_to_code_pointer (void *data) FFI_HIDDEN;
+    static trampoline. */
+ int ffi_tramp_is_present (void *closure) FFI_HIDDEN;
+ 
++/* Return a file descriptor of a temporary zero-sized file in a
++   writable and executable filesystem. */
++int open_temp_exec_file(void) FFI_HIDDEN;
++
+ /* Extended cif, used in callback from assembly routine */
+ typedef struct
+ {
+diff --git a/src/tramp.c b/src/tramp.c
+index 7e005b054..5f19b557f 100644
+--- a/src/tramp.c
++++ b/src/tramp.c
+@@ -39,6 +39,10 @@
+ #ifdef __linux__
+ #define _GNU_SOURCE 1
+ #endif
++
++#include <ffi.h>
++#include <ffi_common.h>
++
+ #include <stdio.h>
+ #include <unistd.h>
+ #include <stdlib.h>