ldm: fix build

There were two problems:
- because buildPhase is specified directly, preBuild ends up never being
  executed; and
- the source is missing a header, resulting in an undefined reference error

pkgs/os-specific/linux/ldm/default.nix

@@ -19,9 +19,10 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ udev utillinux ];
 
-  preBuild = ''
+  postPatch = ''
     substituteInPlace ldm.c \
       --replace "/mnt/" "${mountPath}"
+    sed '16i#include <sys/stat.h>' -i ldm.c
   '';
 
   buildPhase = "make ldm";