xorg.xdm: fix build w/glibc-2.36

Failing Hydra build: https://hydra.nixos.org/build/193323749

pkgs/servers/x11/xorg/overrides.nix

@@ -164,6 +164,7 @@ self: super:
   });
 
   xdm = super.xdm.overrideAttrs (attrs: {
+    patches = (attrs.patches or []) ++ [ ./xdm-fix-header-inclusion.patch ];
     buildInputs = attrs.buildInputs ++ [ libxcrypt ];
     configureFlags = attrs.configureFlags or [] ++ [
       "ac_cv_path_RAWCPP=${stdenv.cc.targetPrefix}cpp"

pkgs/servers/x11/xorg/xdm-fix-header-inclusion.patch

@@ -0,0 +1,29 @@
+On glibc-2.36 this fails with
+
+    genauth.c:45:12: fatal error: bsd/stdlib.h: No such file or directory
+       45 | #  include <bsd/stdlib.h>
+          |            ^~~~~~~~~~~~~~
+
+This is because the file will be included if HAVE_ARC4RANDOM is true and `__linux__` is set.
+However, this is wrong: arc4random is now defined in glibc-2.36 and thus stdlib.h must be included
+even though HAVE_ARC4RANDOM is true.
+
+diff --git a/xdm/genauth.c b/xdm/genauth.c
+index cd2ad61..74d0ae1 100644
+--- a/xdm/genauth.c
++++ b/xdm/genauth.c
+@@ -40,13 +40,7 @@ from The Open Group.
+ 
+ #include <errno.h>
+ 
+-#ifdef HAVE_ARC4RANDOM
+-# ifdef __linux__
+-#  include <bsd/stdlib.h>
+-# else
+-#  include <stdlib.h>
+-# endif
+-#endif
++#include <stdlib.h>
+ 
+ #include <time.h>
+ #define Time_t time_t