ntirpc: fixup build (missing #include)

https://hydra.nixos.org/build/213259376
No idea why it regressed *now*, but I don't expect it matters.

pkgs/development/libraries/ntirpc/default.nix

@@ -15,6 +15,7 @@ stdenv.mkDerivation rec {
 
   postPatch = ''
     substituteInPlace ntirpc/netconfig.h --replace "/etc/netconfig" "$out/etc/netconfig"
+    sed '1i#include <assert.h>' -i src/work_pool.c
   '';
 
   nativeBuildInputs = [ cmake ];