package/flex: fix GCC 15.x build issue

Fixes:
https://autobuild.buildroot.net/results/924b1015d4b81385409ef00f1a14be3ca1959c8e/

As part of building flex for the target a few files are built for the host,
including a rpl_malloc() implementation containing a malloc() forward
declaration without any function parameters.

GCC 15 defaults to -std=gnu23, which handles function declarations without
parameters differently from earlier C standards leading to compilation
errors:

../lib/malloc.c:6:12: warning: conflicting types for built-in function 'malloc'; expected 'void *(long unsigned int)' [-Wbuiltin-declaration-mismatch]
    6 |      void *malloc ();
      |            ^~~~~~
../lib/malloc.c:5:1: note: 'malloc' is declared in header '<stdlib.h>'
    4 |      #include <sys/types.h>
  +++ |+#include <stdlib.h>
    5 |
../lib/malloc.c: In function 'rpl_malloc':
../lib/malloc.c:16:15: error: too many arguments to function 'malloc'; expected 0, have 1

https://gcc.gnu.org/gcc-15/porting_to.html#c23-fn-decls-without-parameters

Add a patch submitted upstream to correct the prototype.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/flex/0004-Match-malloc-signature-to-its-use.patch

@@ -0,0 +1,27 @@
+From 4b142954b54a57a9b0af0a9661056a9c39a8fa95 Mon Sep 17 00:00:00 2001
+From: Richard Barnes <rbarnes@umn.edu>
+Date: Wed, 2 Oct 2024 10:35:09 -0700
+Subject: [PATCH] Match `malloc` signature to its use
+
+Upstream: https://github.com/westes/flex/pull/674
+Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
+---
+ lib/malloc.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/lib/malloc.c b/lib/malloc.c
+index 75e8ef9..701b9b3 100755
+--- a/lib/malloc.c
++++ b/lib/malloc.c
+@@ -3,7 +3,7 @@
+      
+      #include <sys/types.h>
+      
+-     void *malloc ();
++     void *malloc (size_t n);
+      
+      /* Allocate an N-byte block of memory from the heap.
+         If N is zero, allocate a 1-byte block.  */
+-- 
+2.39.5
+